cc: fix another data race in RenderingStatsInstrumentation.

cc: fix another data race in RenderingStatsInstrumentation.

Access to the stats was not protected by a lock. Fix by returning a copy to
the stats and locking while making the copy.

R=enne@chromium.org
BUG=370244
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271646

[ernstm, 2014-05-16 23:19:45]

PTAL

[enne (OOO), 2014-05-19 17:24:45]

lgtm

[ernstm, 2014-05-19 21:56:18]

The CQ bit was checked by ernstm@chromium.org

[commit-bot: I haz the power, 2014-05-19 21:57:02]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ernstm@chromium.org/289163004/1

[commit-bot: I haz the power, 2014-05-20 01:20:26]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-20 04:02:35]

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

[commit-bot: I haz the power, 2014-05-20 04:02:36]

Try jobs failed on following builders:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[ernstm, 2014-05-20 04:32:31]

The CQ bit was checked by ernstm@chromium.org

[commit-bot: I haz the power, 2014-05-20 04:33:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ernstm@chromium.org/289163004/1

[commit-bot: I haz the power, 2014-05-20 15:17:28]

Change committed as 271646