Revert of SkThreadPool ~~> SkTaskGroup

Revert of SkThreadPool ~~> SkTaskGroup (patchset #5 id:80001 of https://codereview.chromium.org/531653002/)

Reason for revert:
It's blocking the DEPS roll.  Error in linux_chromium_gn_rel is:

ERROR at //skia/BUILD.gn:330:3: Item not found
  "//third_party/skia/src/utils/SkThreadPool.h",
  ^--------------------------------------------
You were trying to remove "//third_party/skia/src/utils/SkThreadPool.h"
from the list but it wasn't there.

Original issue's description:
> SkThreadPool ~~> SkTaskGroup
> 
> SkTaskGroup is like SkThreadPool except the threads stay in
> one global pool.  Each SkTaskGroup itself is tiny (4 bytes)
> and its wait() method applies only to tasks add()ed to that
> instance, not the whole thread pool.
> 
> This means we don't need to bring up new thread pools when
> tests themselves want to use multithreading (e.g. pathops,
> quilt).  We just create a new SkTaskGroup and wait for that
> to complete.  This should be more efficient, and allow us
> to expand where we use threads to really latency sensitive
> places.  E.g. we can probably now use these in nanobench
> for CPU .skp rendering.
> 
> Now that all threads are sharing the same pool, I think we
> can remove most of the custom mechanism pathops tests use
> to control threading.  They'll just ride on the global pool
> with all other tests now.
> 
> This (temporarily?) removes the GPU multithreading feature
> from DM, which we don't use.
> 
> On my desktop, DM runs a little faster (57s -> 55s) in
> Debug, and a lot faster in Release (36s -> 24s).  The bots 
> show speedups of similar proportions, cutting more than a
> minute off the N4/Release and Win7/Debug runtimes. 
> 
> BUG=skia:
> 
> Committed: https://skia.googlesource.com/skia/+/9c7207b5dc71dc5a96a2eb107d401133333d5b6f
> 
> Committed: https://skia.googlesource.com/skia/+/406654be7a930b484159f5bca107d3b11d8a9ede

TBR=caryclark@google.com,bsalomon@google.com,bungeman@google.com,mtklein@google.com,reed@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

[jvanverth1, 2014-09-04 13:25:46]

Created Revert of SkThreadPool ~~> SkTaskGroup

[commit-bot: I haz the power, 2014-09-04 13:26:45]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/jvanverth@google.com/540013002/1

[mtklein, 2014-09-04 13:26:56]

The CQ bit was unchecked by mtklein@google.com

[commit-bot: I haz the power, 2014-09-04 13:27:00]

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

[mtklein, 2014-09-04 13:27:24]

On 2014/09/04 13:26:45, I haz the power (commit-bot) wrote:
> CQ is trying da patch. Follow status at
>  https://skia-tree-status.appspot.com/cq/jvanverth@google.com/540013002/1

https://codereview.chromium.org/537363002/

[mtklein, 2014-09-04 13:28:58]

On 2014/09/04 13:27:24, mtklein wrote:
> On 2014/09/04 13:26:45, I haz the power (commit-bot) wrote:
> > CQ is trying da patch. Follow status at
> >  https://skia-tree-status.appspot.com/cq/jvanverth@google.com/540013002/1
> 
> https://codereview.chromium.org/537363002/

We can't fix this problem by rolling back.  We'd just have to reland it and
manually roll anyway.  So let's manually roll now without rolling it back.