Fix crash on suspend for chrome os

Fix crash on suspend for chrome os

If RendererFreezer::SuspendImminent was called multiple times before it
had a chance to report readiness, it would post multiple tasks to do so
on the current message loop.  However the callback to report readiness
would be cleared after the first task ran and would cause a crash when
the next task tried to run it.  Fix this by not posting new tasks while
a callback is pending and just update the callback instead.

BUG=414396
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>;

Committed: https://crrev.com/1a1fd18086ffee9f111b4454098665d1c9fc0b4b
Cr-Commit-Position: refs/heads/master@{#294916}

[Chirantan Ekbote, 2014-09-15 21:09:39]

chirantan@chromium.org changed reviewers:
+ derat@chromium.org

[Chirantan Ekbote, 2014-09-15 21:09:39]

ptal

[Daniel Erat, 2014-09-15 21:13:21]

lgtm

[Chirantan Ekbote, 2014-09-15 21:14:18]

The CQ bit was checked by chirantan@chromium.org

[commit-bot: I haz the power, 2014-09-15 21:15:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/550093004/1

[commit-bot: I haz the power, 2014-09-15 23:26:02]

Committed patchset #1 (id:1) as 3e8b90c518783a5a888989cfdae75690d4059e1d

[commit-bot: I haz the power, 2014-09-15 23:32:19]

Patchset 1 (id:??) landed as
https://crrev.com/1a1fd18086ffee9f111b4454098665d1c9fc0b4b
Cr-Commit-Position: refs/heads/master@{#294916}