tracing: split out the CategoryRegistry from the TraceLog

tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689
Committed: https://crrev.com/094ac7f87d78a80e3934e148855a908977559166
Cr-Commit-Position: refs/heads/master@{#431300}

[Primiano Tucci (use gerrit), 2016-10-26 15:11:05]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-26 15:11:20]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-26 15:19:23]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-26 15:19:23]

Dry run: Try jobs failed on following builders:
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Primiano Tucci (use gerrit), 2016-10-26 15:40:09]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-26 15:40:30]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-26 15:49:04]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-26 15:49:05]

Dry run: Try jobs failed on following builders:
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Primiano Tucci (use gerrit), 2016-10-26 16:26:58]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[Primiano Tucci (use gerrit), 2016-10-26 16:27:04]

Patchset #2 (id:20001) has been deleted

[commit-bot: I haz the power, 2016-10-26 16:27:26]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Primiano Tucci (use gerrit), 2016-10-26 16:31:18]

Description was changed from

==========
WIP category

BUG=
==========

to

==========
tracing: split out the CategoryRegistry from the TraceLog

This is a refactoring in light of the upcoming tracing reorg/unbundling.
This CL splits out the category logic out of the TraceLog and 
allows it to be more easily reused.
Furthermore this CL takes the occasion for some naming cleanup:
"category group" has always been a confusing name and all it does is
reflecting a mere and subtle implementation detail (the fact that a 
category name can be "cat1,cat2") which is quite irrelevant from a 
functional viewpoint.

BUG=659689
==========

[Primiano Tucci (use gerrit), 2016-10-26 16:36:25]

Description was changed from

==========
tracing: split out the CategoryRegistry from the TraceLog

This is a refactoring in light of the upcoming tracing reorg/unbundling.
This CL splits out the category logic out of the TraceLog and 
allows it to be more easily reused.
Furthermore this CL takes the occasion for some naming cleanup:
"category group" has always been a confusing name and all it does is
reflecting a mere and subtle implementation detail (the fact that a 
category name can be "cat1,cat2") which is quite irrelevant from a 
functional viewpoint.

BUG=659689
==========

to

==========
tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689
==========

[Primiano Tucci (use gerrit), 2016-10-26 16:52:37]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-26 16:52:58]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Primiano Tucci (use gerrit), 2016-10-26 17:01:54]

primiano@chromium.org changed reviewers:
+ oysteine@chromium.org

[Primiano Tucci (use gerrit), 2016-10-26 17:01:55]

allright, let's start some autumn cleanup.
Looks like doing the non-categories might be easier than expected.

[commit-bot: I haz the power, 2016-10-26 17:18:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-26 17:19:00]

Dry run: Try jobs failed on following builders:
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)

[Primiano Tucci (use gerrit), 2016-10-26 17:31:11]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-26 17:32:21]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[fmeawad, 2016-10-26 17:35:59]

fmeawad@chromium.org changed reviewers:
+ fmeawad@chromium.org

[fmeawad, 2016-10-26 17:36:00]

Concerning the naming cleanup, I don't think that category group is confusing.
In my opinion: cat1,cat2 is a group of 2 categories cat1 and cat2. I think
calling cat1,cat2 a category may result in later confusion when we are
refactoring the traceconfig subsystem

Otherwise, super excited for this refactor.

[commit-bot: I haz the power, 2016-10-26 17:59:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-26 17:59:59]

Dry run: Try jobs failed on following builders:
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)

[Primiano Tucci (use gerrit), 2016-10-27 15:38:27]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-27 15:38:45]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-27 16:54:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-27 16:54:45]

Dry run: Try jobs failed on following builders:
  mac_chromium_rel_ng on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[oystein (OOO til 10th of July), 2016-10-28 00:40:19]

On 2016/10/26 17:36:00, fmeawad wrote:
> Concerning the naming cleanup, I don't think that category group is confusing.
> In my opinion: cat1,cat2 is a group of 2 categories cat1 and cat2. I think
> calling cat1,cat2 a category may result in later confusion when we are
> refactoring the traceconfig subsystem
> 
> Otherwise, super excited for this refactor.

I'm a little bit conflicted about this. To some degree, on the tracelog level
there's really just "category string" with no knowledge of what's actually in
that string and how it's used. The knowledge that there's such a thing as
comma-separated categories and you can tell the config you just need to match
one of them, is strictly defined by the TraceConfig. It's opaque to the other
parts of the system, and if the end result of all of this is that TraceConfig
becomes an impl-level thing and could differ between implementations that may
matter. But I don't think it's right to call it "category" either.
"CategoryConfig"?

[oystein (OOO til 10th of July), 2016-10-28 06:28:00]

partial first pass

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
File base/trace_event/category_registry.cc (right):

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:34: base::LazyInstance<base::Lock>::Leaky
g_category_lock =
Lazily created locks scare me :). Are we sure there's no races for the lock
creation itself?

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:37: void DCheckIsValidCategoryPtr(const
TraceCategory* category) {
#if DCHECK_IS_ON() rather than "DCheck" as part of the name, both around the
definition and the callsites, might be clearer (and more verbose, I realize, but
I think that's the pattern generally in use).

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:48: TraceCategory* const
CategoryRegistry::kCategoryExhausted = &g_categories[0];
Why are these outside of the anonymous namespace?

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:146: return p <
reinterpret_cast<uintptr_t>(&g_categories[kNumBuiltinCategories]);
Hmm do we actually need all o the uintptr_t casting? I thought pointers to
within the same array could be compared directly.

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
File base/trace_event/category_registry.h (right):

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.h:29: : begin_(begin), end_(end) {}
Can we DCHECK(begin_ <= end_)?

[Primiano Tucci (use gerrit), 2016-10-28 15:28:39]

On 2016/10/28 00:40:19, oystein wrote:
> On 2016/10/26 17:36:00, fmeawad wrote:
> > Concerning the naming cleanup, I don't think that category group is
confusing.
> > In my opinion: cat1,cat2 is a group of 2 categories cat1 and cat2. I think
> > calling cat1,cat2 a category may result in later confusion when we are
> > refactoring the traceconfig subsystem
> > 
> > Otherwise, super excited for this refactor.
> 
> I'm a little bit conflicted about this. To some degree, on the tracelog level
> there's really just "category string" with no knowledge of what's actually in
> that string and how it's used. The knowledge that there's such a thing as
> comma-separated categories and you can tell the config you just need to match
> one of them, is strictly defined by the TraceConfig. It's opaque to the other
> parts of the system, and if the end result of all of this is that TraceConfig
> becomes an impl-level thing and could differ between implementations that may
> matter. But I don't think it's right to call it "category" either.
> "CategoryConfig"?

I agree with Oystein here. The "comma" behavior is totally a feature of the
TraceConfig not the rest of the trace machinery.
Nothing here cares about commas. From a more practical view, the usage of
category "groups" is ~6% (71 vs 1057, see [1,2])
so I'd say it's a pretty unusual feature.

Happy to find a different name which is not just "Category".
CategoryConfig though smells too close to TraceConfig.
Any other proposal?

[1]
https://cs.chromium.org/search/?q=TRACE_EVENT%5Cw%2B%5C(%5C%22%5B%5E%22%5D%2B...
[2]
https://cs.chromium.org/search/?q=TRACE_EVENT%5Cw%2B%5C(%5C%22&sq=package:chr...

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
File base/trace_event/category_registry.cc (right):

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:34: base::LazyInstance<base::Lock>::Leaky
g_category_lock =
On 2016/10/28 06:28:00, oystein wrote:
> Lazily created locks scare me :). Are we sure there's no races for the lock
> creation itself?

This is a quite established pattern:
https://cs.chromium.org/search/?q=LazyInstance%3Cbase::Lock%3E&sq=package:chr...

The problem is the following: in theory I'd want just a base::Lock here. I can't
just do that because that would cause a static initializer. So I wrap it in a
LazyInstance.
LazyInstance itself has the right barriers to gurantee threadsafe
initialization. See comment in base/lazy_instance.h

Having said that, I was thinking to rip out this lock at some point before the
unbundling and just use a spinlock with atomics. I don't want to have to port
LazyInstance in whatever_will_be/core and the critical section here is really
trivial, as we don't do any initialization, just a bunch of strcmp.

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:37: void DCheckIsValidCategoryPtr(const
TraceCategory* category) {
On 2016/10/28 06:28:00, oystein wrote:
> #if DCHECK_IS_ON() rather than "DCheck" as part of the name, both around the
> definition and the callsites, might be clearer (and more verbose, I realize,
but
> I think that's the pattern generally in use).

Actually thinking more I think the right pattern is:
- this function just being a "bool IsValidCategory".
- use DCHECK(IsValidCategory(...))

makes both sense from a readability viewpoint and also is the pattern used by 
DCHECK(thread_checker_.CalledOnValidThread());

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:48: TraceCategory* const
CategoryRegistry::kCategoryExhausted = &g_categories[0];
On 2016/10/28 06:28:00, oystein wrote:
> Why are these outside of the anonymous namespace?

because I want to be able to reference to some of these in the code.
For instance kCategoryMetadata is referenced by trace_log.cc
This is the way we will get rid of the deferred static category_group_enabled f
checks.
Now you can just have the compiler initialize the category_group_enabled in the
macros by just using these objects.I just need to change the macros from caching
the ptr to the word, to the ptr of the full TraceCategory object (which is
really the same pointer, because I guarantee that the enabled flag is the first
field of the TraceCategory). Will do in some next CLs.

In theory I could move kCategoryExhausted and kCategoryAlreadyShutdown to anon
namespace and leave kMetadata here. But IMHO it feels more readable if they are
all in the same place one after the other.

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.cc:146: return p <
reinterpret_cast<uintptr_t>(&g_categories[kNumBuiltinCategories]);
On 2016/10/28 06:28:00, oystein wrote:
> Hmm do we actually need all o the uintptr_t casting? I thought pointers to
> within the same array could be compared directly.

good point. you are right. done.

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
File base/trace_event/category_registry.h (right):

https://codereview.chromium.org/2452063003/diff/120001/base/trace_event/categ...
base/trace_event/category_registry.h:29: : begin_(begin), end_(end) {}
On 2016/10/28 06:28:00, oystein wrote:
> Can we DCHECK(begin_ <= end_)?

Done.

[Primiano Tucci (use gerrit), 2016-10-28 15:28:46]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-28 15:29:05]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-28 16:44:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-28 16:44:38]

Dry run: Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Primiano Tucci (use gerrit), 2016-10-31 13:36:07]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-31 13:36:27]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-31 13:42:47]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-31 13:42:48]

Dry run: Try jobs failed on following builders:
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[Primiano Tucci (use gerrit), 2016-10-31 13:50:03]

Patchset #8 (id:160001) has been deleted

[Primiano Tucci (use gerrit), 2016-10-31 13:53:55]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-31 13:54:11]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-31 14:00:38]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-31 14:00:39]

Dry run: Try jobs failed on following builders:
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Primiano Tucci (use gerrit), 2016-11-01 10:03:01]

Patchset #5 (id:100001) has been deleted

[Primiano Tucci (use gerrit), 2016-11-01 10:03:10]

Patchset #4 (id:80001) has been deleted

[Primiano Tucci (use gerrit), 2016-11-01 10:03:12]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[Primiano Tucci (use gerrit), 2016-11-01 10:03:19]

Patchset #1 (id:1) has been deleted

[commit-bot: I haz the power, 2016-11-01 10:03:22]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Primiano Tucci (use gerrit), 2016-11-01 10:03:23]

Patchset #1 (id:40001) has been deleted

[Primiano Tucci (use gerrit), 2016-11-01 10:03:33]

Patchset #1 (id:60001) has been deleted

[commit-bot: I haz the power, 2016-11-01 11:15:07]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-01 11:15:08]

Dry run: This issue passed the CQ dry run.

[Primiano Tucci (use gerrit), 2016-11-01 12:02:58]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-11-01 12:03:07]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-01 13:04:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-01 13:04:18]

Dry run: This issue passed the CQ dry run.

[Primiano Tucci (use gerrit), 2016-11-01 15:07:54]

Ok this seems ready to go % name of TraceCategory if anybody wants to come up
with something else.
Fun times with compilers
(https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/688aPLsA5jU)

[fmeawad, 2016-11-01 20:48:32]

Was the old code tested and we are reusing the tests? are we relying on some end
to end testing, or is it worth it adding tests here?

[Primiano Tucci (use gerrit), 2016-11-04 23:11:25]

The CQ bit was checked by primiano@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-11-04 23:12:13]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Primiano Tucci (use gerrit), 2016-11-04 23:12:42]

On 2016/11/01 20:48:32, fmeawad wrote:
> Was the old code tested and we are reusing the tests? are we relying on some
end
> to end testing, or is it worth it adding tests here?

Yup this code is still covered by the old tests. However extra tests are never
bad, so I just added new ones in the latest patchset.

[commit-bot: I haz the power, 2016-11-05 00:57:10]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-05 00:57:11]

Dry run: This issue passed the CQ dry run.

[Primiano Tucci (use gerrit), 2016-11-07 15:15:12]

ping.

[fmeawad, 2016-11-07 15:48:58]

Thanks, lgtm

[Primiano Tucci (use gerrit), 2016-11-10 10:29:25]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2016-11-10 10:29:37]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-10 10:36:21]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-10 10:36:22]

Try jobs failed on following builders:
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Primiano Tucci (use gerrit), 2016-11-10 12:51:52]

primiano@chromium.org changed reviewers:
+ thakis@chromium.org

[Primiano Tucci (use gerrit), 2016-11-10 12:51:53]

ehm, +thakis for base/BUILD.gn stamp

[Nico, 2016-11-10 15:51:51]

lgtm

[Primiano Tucci (use gerrit), 2016-11-10 17:56:30]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2016-11-10 17:57:04]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-10 18:43:42]

Description was changed from

==========
tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689
==========

to

==========
tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689
==========

[commit-bot: I haz the power, 2016-11-10 18:43:44]

Committed patchset #6 (id:240001)

[commit-bot: I haz the power, 2016-11-10 19:09:04]

Description was changed from

==========
tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689
==========

to

==========
tracing: split out the CategoryRegistry from the TraceLog

Changes introduced by this CL:
 - Split out the category logic out of the TraceLog into CategoryRegistry.
   Makes it easier to unbundle, reuse and reason about.

 - Get rid of the parallel arrays for tracking names, filters and state
   of each category and switch them to one array of struct. This not
   only makes the code easier to read, but also opens the way for
   having categories defined at compile time that don't need any
   lazy initialization of the "enabled" ptr.

 - Naming cleanup: the term "category group" has always been confusing.
   All it does is reflecting a subtle implementation detail (the fact
   that a category name can be "cat1,cat2") which is quite irrelevant
   from a functional viewpoint.

BUG=659689

Committed: https://crrev.com/094ac7f87d78a80e3934e148855a908977559166
Cr-Commit-Position: refs/heads/master@{#431300}
==========

[commit-bot: I haz the power, 2016-11-10 19:09:06]

Patchset 6 (id:??) landed as
https://crrev.com/094ac7f87d78a80e3934e148855a908977559166
Cr-Commit-Position: refs/heads/master@{#431300}

[oystein (OOO til 10th of July), 2016-11-10 22:02:13]

lgtm!