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With this CL, WaitableEventWatcher uses SequencedTaskRunnerHandle
instead of MessageLoop::current() to post back to the sequence that
called StartWatching().

Also, WaitableEventWatcher no longer registers itself as a destruction
observer of the MessageLoop from which StartWatching() is called. If
the watched WaitableEvent is signaled after the MessageLoop is
destroyed, the task posted by WaitableEventWatcher to the
SequencedTaskRunner will simply not run (no crash).

MessageLoop::current() and destruction observer do not work with
TaskScheduler.

BUG=650318

Review-Url: https://codereview.chromium.org/2368423002
Cr-Commit-Position: refs/heads/master@{#422856}