Reland of [tracing] Display the locked size of discardable memory segment.

Reland of [tracing] Display the locked size of discardable memory segment. (patchset #1 id:1 of https://codereview.chromium.org/1386333003/ )

Reason for revert:
Reland after the fix. The crash was due to IsMemoryLocked called for segments 0
which have no underlying shared memory, causing dcheck to fail. Fixed by
removing not dumping the empty segments.

Original issue's description:
> Revert of [tracing] Display the locked size of discardable memory segment. (patchset #8 id:200001 of https://codereview.chromium.org/1374213002/ )
>
> Reason for revert:
> Speculative revert for the bug: www.crbug.com/541029
>
> Original issue's description:
> > [tracing] Display the locked size of discardable memory segment.
> >
> > The locked size of the discardable memory segments is now displayed on
> > tracing. The browser side can only tell if the whole segment is locked
> > or not, if the segment is shared with a child. The child side can also
> > tell how much memory in the segment is locked.
> >
> > BUG=529943
> >
> > Committed: https://crrev.com/a29f3c7b772a058c69b91d393740b85d7d23526b
> > Cr-Commit-Position: refs/heads/master@{#352004}
>
> TBR=reveman@chromium.org,avi@chromium.org,thestig@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=529943
>
> Committed: https://crrev.com/759242daa7de8683f62885673b5dfba75679727b
> Cr-Commit-Position: refs/heads/master@{#353260}

TBR=thestig@chromium.org
BUG=529943
Committed: https://crrev.com/b16ac46e052b3daab95cd6518916579b80aecabb
Cr-Commit-Position: refs/heads/master@{#353833}

[ssid, 2015-10-13 14:57:10]

Created Reland of [tracing] Display the locked size of discardable memory
segment.

[ssid, 2015-10-13 15:19:25]

reveman@ Can you take a look?

[ssid, 2015-10-13 15:47:01]

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

[commit-bot: I haz the power, 2015-10-13 16:00:36]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1407483003/200001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1407483003/200001

[commit-bot: I haz the power, 2015-10-13 18:05:43]

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

[commit-bot: I haz the power, 2015-10-13 18:05:43]

Dry run: This issue passed the CQ dry run.

[reveman, 2015-10-13 18:24:33]

lgtm with nit but let's land this without changing that as it has passed the cq
dry run.

https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
File content/common/discardable_shared_memory_heap.cc (right):

https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
content/common/discardable_shared_memory_heap.cc:372: size_t
allocated_objects_blocks = 0;
nit: /_objects_blocks/objects_size_in_blocks/

https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
content/common/discardable_shared_memory_heap.cc:373: size_t
locked_objects_blocks = 0;
nit: ditto

[ssid, 2015-10-13 18:47:57]

On 2015/10/13 18:24:33, reveman wrote:
> lgtm with nit but let's land this without changing that as it has passed the
cq
> dry run.
> 
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> File content/common/discardable_shared_memory_heap.cc (right):
> 
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> content/common/discardable_shared_memory_heap.cc:372: size_t
> allocated_objects_blocks = 0;
> nit: /_objects_blocks/objects_size_in_blocks/
> 
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> content/common/discardable_shared_memory_heap.cc:373: size_t
> locked_objects_blocks = 0;
> nit: ditto

Okay, I will upload a follow up CL for this reason.

[Avi (use Gerrit), 2015-10-13 19:37:22]

On 2015/10/13 18:47:57, ssid wrote:
> On 2015/10/13 18:24:33, reveman wrote:
> > lgtm with nit but let's land this without changing that as it has passed the
> cq
> > dry run.
> > 
> >
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> > File content/common/discardable_shared_memory_heap.cc (right):
> > 
> >
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> > content/common/discardable_shared_memory_heap.cc:372: size_t
> > allocated_objects_blocks = 0;
> > nit: /_objects_blocks/objects_size_in_blocks/
> > 
> >
>
https://codereview.chromium.org/1407483003/diff/200001/content/common/discard...
> > content/common/discardable_shared_memory_heap.cc:373: size_t
> > locked_objects_blocks = 0;
> > nit: ditto
> 
> Okay, I will upload a follow up CL for this reason.

do it. lgtm

[ssid, 2015-10-13 19:49:45]

The CQ bit was checked by ssid@chromium.org

[commit-bot: I haz the power, 2015-10-13 19:52:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1407483003/200001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1407483003/200001

[commit-bot: I haz the power, 2015-10-13 20:10:34]

Committed patchset #2 (id:200001)

[commit-bot: I haz the power, 2015-10-13 20:12:53]

Patchset 2 (id:??) landed as
https://crrev.com/b16ac46e052b3daab95cd6518916579b80aecabb
Cr-Commit-Position: refs/heads/master@{#353833}