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The Lock is causing hangs because of priority inversion
mixed with priority boosting (ScheduleWork() tends to
boost the destination thread which may deschedule the
posting thread; if the posting thread is a background
thread this boost-induded-desched-while-holding-lock
can cause a livelock). See https://crbug.com/890978#c10
for example crashes catching this.

The Lock was only necessary for startup/shutdown and is
being replaced by a lock-free atomic scheme in this CL.

MessagePump::ScheduleWork() itself was already thread-safe
(but the Android impl did unnecessarily check a non-atomic bool)

This adds a WaitableEvent in ~MessageLoop(); hence the requirement
for a wait-allowance in net's EmbeddedTestServer.

TBR=zhongyi@chromium.org (embedded_test_server.cc side-effects)

Bug: 890978, 874237
Change-Id: I0916e5a99035a935b0a23a770af256f334e78c43
Reviewed-on: https://chromium-review.googlesource.com/c/1278631
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601600}