Add subtract to atomics.

Add subtract to atomics.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/3667d5bf3f6d8eea90142046202c4f61cfcd5fea

[herb_g, 2015-09-16 14:01:27]

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

[herb_g, 2015-09-16 14:02:01]

There will be a follow on CL to update SharedMutex to use this new atomic.

[mtklein, 2015-09-16 14:28:19]

lgtm, though I'm inclined to think the need for fetch_sub means we might be
kidding ourselves about some field that's unsigned/size_t/uintN_t that really
should just be switched to int or int64_t.

[herb_g, 2015-09-16 14:31:00]

The CQ bit was checked by herb@google.com

[commit-bot: I haz the power, 2015-09-16 14:31:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1348113004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1348113004/1

[commit-bot: I haz the power, 2015-09-16 14:32:16]

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

[commit-bot: I haz the power, 2015-09-16 14:32:17]

Try jobs failed on following builders:
  skia_presubmit-Trybot on client.skia.fyi (JOB_FAILED,
http://build.chromium.org/p/client.skia.fyi/builders/skia_presubmit-Trybot/bu...)

[herb_g, 2015-09-16 14:40:43]

You are right for when the atomic is the entire datum, but if the datum is split
into fields, then this can be useful.

[herb_g, 2015-09-16 14:41:12]

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

[mtklein, 2015-09-16 14:42:21]

On 2015/09/16 14:40:43, herb_g wrote:
> You are right for when the atomic is the entire datum, but if the datum is
split
> into fields, then this can be useful.

That's where I'm skeptical that add and subtract are good things to be doing. :P

[reed1, 2015-09-16 14:43:47]

lgtm

[herb_g, 2015-09-16 14:45:37]

The CQ bit was checked by herb@google.com

[commit-bot: I haz the power, 2015-09-16 14:45:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1348113004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1348113004/1

[commit-bot: I haz the power, 2015-09-16 14:46:22]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/3667d5bf3f6d8eea90142046202c4f61cfcd5fea