Fix DiscardableMemoryPool::free race condition

Fix DiscardableMemoryPool::free race condition

BUG=skia:3596
Committed: https://skia.googlesource.com/skia/+/2edf599eebd370d5a95cafb2a668c4e9e0e9c094

[hal.canary, 2015-03-26 20:53:29]

halcanary@google.com changed reviewers:
+ mtklein@google.com, reed@google.com

[hal.canary, 2015-03-26 20:53:30]

PTAL

[hal.canary, 2015-03-26 20:53:33]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2015-03-26 20:53:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1017943003/1

[commit-bot: I haz the power, 2015-03-26 20:53:46]

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 2015-03-27 02:53 UTC

[mtklein, 2015-03-26 20:54:35]

Ah, lgtm.  Please wait for reed@ too.

[mtklein, 2015-03-26 20:54:39]

The CQ bit was unchecked by mtklein@google.com

[hal.canary, 2015-03-26 20:54:54]

halcanary@google.com changed required reviewers:
+ reed@google.com

[reed1, 2015-03-26 21:08:34]

The CQ bit was checked by reed@google.com

[reed1, 2015-03-26 21:08:35]

lgtm

[commit-bot: I haz the power, 2015-03-26 21:08:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1017943003/1

[commit-bot: I haz the power, 2015-03-26 21:09:01]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/2edf599eebd370d5a95cafb2a668c4e9e0e9c094