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This is a reland of f7da13a2
(original change in PS1)

The original change was missing acquire ordering in
DisconnectFromParent(). This is necessary in order for
the disconnecting thread to see all memory side-effects
previously made by other threads (or some side-effects
of message_loop_->ScheduleWork() could racily come in
after ~MessageLoop()).

Also removed the DCHECK that
|operations_state_ == kDisconnectedBit| at the end of
DisconnectFromParent() as it was incorrect. A racy
BeforeOperation() call can make it 1->3 (and no-op) after
DisconnectFromParent() made it 0->1.

And lastly, added a scoped allowance to always allow the
very fast wait instead of requiring callers to know about
this implementation detail of MessageLoop (and reverted
changes to //net).

Original change's description:
> [MessageLoop] Lock-free ScheduleWork() scheme
>
> 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}

Bug: 890978, 874237, 897925
Change-Id: I17c515f9a3169bbdfc303a4b259f34097e31730d
Reviewed-on: https://chromium-review.googlesource.com/c/1297129Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602166}