Use atomic subtract in shared mutex.

Use atomic subtract in shared mutex.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/3060f3efa0d15f613eeeb25221f2b9bf728cc8aa

[herb_g, 2015-09-16 14:59:02]

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

[commit-bot: I haz the power, 2015-09-16 14:59:15]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1348173003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1348173003/1

[herb_g, 2015-09-16 15:00:03]

herb@google.com changed reviewers:
+ mtklein@google.com

[commit-bot: I haz the power, 2015-09-16 15:05:23]

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

[commit-bot: I haz the power, 2015-09-16 15:05:24]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-09-16 15:20:31]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-09-16 15:20:31]

lgtm

[commit-bot: I haz the power, 2015-09-16 15:20:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1348173003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1348173003/1

[commit-bot: I haz the power, 2015-09-16 15:21:17]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/3060f3efa0d15f613eeeb25221f2b9bf728cc8aa