fix race to set the new gen ID

Fix the race to set the new gen ID.

There's a small window where two threads can both see the gen ID is zero,
both go get new distinct genIDs, but race to write them.

This now uses compare_exchange to read and leave the winner's ID in place
when there's a race.

SkAtomics isn't public.
TBR=reed@google.com

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/59c9203321cf4afb48597f39e4bdd4ae91da12a0

[mtklein_C, 2015-02-25 17:19:37]

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

[reed1, 2015-02-25 17:26:13]

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

[reed1, 2015-02-25 17:26:13]

ben, can you take a peek?

[mtklein, 2015-02-25 18:17:36]

mtklein@google.com changed required reviewers:
+ bungeman@google.com

[mtklein, 2015-02-25 18:17:42]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-02-25 18:18:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956013002/60001

[commit-bot: I haz the power, 2015-02-25 18:18:19]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-02-26 00:18 UTC

[commit-bot: I haz the power, 2015-02-25 18:18:23]

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

[commit-bot: I haz the power, 2015-02-25 18:18:24]

Presubmit check for 956013002-60001 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Since the CL is editing public API, you must have an LGTM from one of:
('reed@chromium.org', 'reed@google.com', 'bsalomon@chromium.org',
'bsalomon@google.com', 'djsollen@chromium.org', 'djsollen@google.com')

Was the presubmit check useful? If not, run "git cl presubmit -v"
to figure out which PRESUBMIT.py was run, then run git blame
on the file to figure out who to ask for help.

[bungeman-skia, 2015-02-25 19:43:01]

lgtm, if it works

[mtklein, 2015-02-25 20:34:48]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-02-25 20:35:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956013002/60001

[commit-bot: I haz the power, 2015-02-25 20:52:00]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/59c9203321cf4afb48597f39e4bdd4ae91da12a0