Add lock support for in process context provider.

Add lock support for in process context provider.

The context lost callback will be called on the thread that the command buffer was bound to, and with the lock held.

BUG=454500
Committed: https://crrev.com/7dc2605574f04851cc6d2bc000a5a65e055d7e56
Cr-Commit-Position: refs/heads/master@{#316341}

[jbauman, 2015-02-13 21:08:29]

jbauman@chromium.org changed reviewers:
+ piman@chromium.org

[piman, 2015-02-13 22:02:54]

lgtm

[vmiura, 2015-02-13 22:13:11]

The CQ bit was checked by vmiura@chromium.org

[commit-bot: I haz the power, 2015-02-13 22:13:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/917223003/20001

[commit-bot: I haz the power, 2015-02-13 23:14:43]

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

[commit-bot: I haz the power, 2015-02-13 23:14:44]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[vmiura, 2015-02-13 23:20:43]

The CQ bit was checked by vmiura@chromium.org

[commit-bot: I haz the power, 2015-02-13 23:22:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/917223003/20001

[commit-bot: I haz the power, 2015-02-14 00:23:02]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-14 00:23:34]

Patchset 2 (id:??) landed as
https://crrev.com/7dc2605574f04851cc6d2bc000a5a65e055d7e56
Cr-Commit-Position: refs/heads/master@{#316341}