Modernize atomics in SkTaskGroup's threadpool.

Modernize atomics in SkTaskGroup's threadpool.

    - Use SkAtomic<int32_t> for pending work count so we're statically
      forced to operate on it with atomic methods.
    - Replacing old methods like sk_atomic_inc/dec gives us finer control
      over which barriers we need for each operation.

No public API changes.
TBR=reed@google.com

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/942e99b9c411d093851db8d12c8357f409563007

[mtklein_C, 2015-06-17 12:39:21]

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

[mtklein, 2015-06-17 12:41:15]

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

[commit-bot: I haz the power, 2015-06-17 12:41:24]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1193493003/1

[commit-bot: I haz the power, 2015-06-17 12:46:48]

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

[commit-bot: I haz the power, 2015-06-17 12:46:48]

Dry run: This issue passed the CQ dry run.

[bungeman-skia, 2015-06-17 14:30:29]

Unless you're being really sneaky, this looks like it's doing the same thing it
did before. lgtm

[mtklein, 2015-06-17 14:52:43]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-06-17 14:52:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1193493003/1

[commit-bot: I haz the power, 2015-06-17 14:53:27]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/942e99b9c411d093851db8d12c8357f409563007