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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>

Review URL: https://codereview.chromium.org/550093004

Cr-Commit-Position: refs/heads/master@{#294916}