Add ScopedAllowWait to TaskGraphRunner to Avoid Crash in Shutdown

Add ScopedAllowWait to TaskGraphRunner to Avoid Crash in Shutdown

When shutting down, TaskGraphRunner will clear out tasks that are ready
to run and wait for running tasks to finish running. Wait is disallowed
in main thread and crashes in single thread impl-side painting.

R=danakj, jam, brettw
BUG=369653
Committed: https://crrev.com/0471c66927ca0c21a0785174ef3fe245b9908766
Cr-Commit-Position: refs/heads/master@{#318265}

[weiliangc, 2015-02-25 21:47:04]

This CL adds friend class to base::ThreadRestrictions, thus require review from
jam and/or brettw.
Reasoning can be found in
https://code.google.com/p/chromium/issues/detail?id=369653#c31

Thanks.

[jam, 2015-02-26 05:51:10]

lgtm

[danakj, 2015-02-26 17:30:45]

LGTM

[danakj, 2015-02-26 17:30:49]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-02-26 17:31:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/946103004/1

[commit-bot: I haz the power, 2015-02-26 18:33:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-26 18:33:59]

Patchset 1 (id:??) landed as
https://crrev.com/0471c66927ca0c21a0785174ef3fe245b9908766
Cr-Commit-Position: refs/heads/master@{#318265}