cc: Fix WeakPtrFactory::GetWeakPtr() race when using multiple raster worker threads.

cc: Fix WeakPtrFactory::GetWeakPtr() race when using multiple raster worker threads.

This prevents multiple threads from being able to call GetWeakPtr()
at the same time by making sure the lock is acquired when this is
called.

R=vmpstr@chromium.org
BUG=

Committed: https://crrev.com/d130dcddb4bced6c4058d8d2fd6560b41fc5103a
Cr-Commit-Position: refs/heads/master@{#302717}

[vmpstr, 2014-11-04 21:52:23]

lgtm

[reveman, 2014-11-04 22:42:59]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2014-11-04 22:44:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/700843004/20001

[commit-bot: I haz the power, 2014-11-04 22:50:57]

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

[commit-bot: I haz the power, 2014-11-04 22:50:59]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)
  linux_chromium_asan_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  linux_chromium_compile_dbg_32 on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[reveman, 2014-11-04 23:09:08]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2014-11-04 23:10:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/700843004/40001

[commit-bot: I haz the power, 2014-11-05 00:24:05]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-11-05 00:24:46]

Patchset 3 (id:??) landed as
https://crrev.com/d130dcddb4bced6c4058d8d2fd6560b41fc5103a
Cr-Commit-Position: refs/heads/master@{#302717}