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Provide more information by adding a trace event to WaitableEvent::Wait.
Connect WaitableEvent::Signal with the end of WaitableEvent::TimedWait by a flow event.

Make the implementation of WaitableEvent::Wait cross-platform and move
it in waitable_event.cc.

Bug: 1043616
Change-Id: I07d13d43e951d6fa223202ccd5f6a076559b416f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2267160Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Karel Král <karelkral@google.com>
Cr-Commit-Position: refs/heads/master@{#791831}