Add and use SkSemaphore

Add and use SkSemaphore

This allows a faster implementation of our SkTaskGroup thread pool.
It also means we don't need SkCondVar (which, remember, isn't supported on XP.)

Doing some testing with SampleApp, this really cuts down on the overhead from SkTaskGroup, e.g. 30% to 10%.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/61fa22be104a3e98998fa1b24cd47b886df5721c

[mtklein, 2015-06-17 16:16:15]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-17 16:16:21]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192573003/60001

[mtklein, 2015-06-17 16:18:45]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-17 16:18:53]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192573003/80001

[commit-bot: I haz the power, 2015-06-17 16:29:56]

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

[commit-bot: I haz the power, 2015-06-17 16:29:57]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-06-17 16:30:33]

mtklein@chromium.org changed reviewers:
+ bungeman@google.com
- mtklein@google.com

[bungeman-skia, 2015-06-17 17:44:16]

lgtm

[mtklein_C, 2015-06-17 17:44:26]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-06-17 17:44:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192573003/100001

[commit-bot: I haz the power, 2015-06-17 17:50:28]

Committed patchset #6 (id:100001) as
https://skia.googlesource.com/skia/+/61fa22be104a3e98998fa1b24cd47b886df5721c