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This was extracted from
https://chromium-review.googlesource.com/c/chromium/src/+/1088762/17
in an attempt to simplify it (and is now a precursor to it).
(MessageLoop::Controller was IncomingTaskQueue::MessageLoopController
there but after grokking the resulting CL it's simpler under MessageLoop
as done in this CL)

This CL splits the logic in IncomingTaskQueue which took care of
scheduling the MessageLoop into a dedicated class. In that follow-up CL:
MessageLoopTaskRunner will interact directly with the task Observer and
take IncomingTaskQueue completely out of the picture (merely a data
structure holding various task queues at that point).

IncomingTaskQueue also took care of detaching on MessageLoop shutdown
which was moved to the new Controller class as well.

message_loop.cc is the best place for this extracted logic as it all
pertains precisely to MessageLoop's implementation detail (how
ScheduleWork should be invoked).

This CL simplifies locking as well by having a clear separation between
the two locks instead of two locks in the same class used
interchangibly. |incoming_queue_lock_| is now strictly for incoming
tasks. |message_loop_lock_| is now strictly for
ScheduleWork()/DisconnectFromParent().

Note: |message_loop_scheduled_| was dropped as it was redundant (always
equal to |!was_empty|).

Performance wise, the perf tests show that this change is a noop :
 * While BasicPostTaskPerfTest became simpler (executed less code) with
   this CL :
     The new BasicPostTaskPerfTest w/ MockObserverSimulatingOverhead
     reintroduces that overhead to show that it's still the same (or
     slightly in favor of this CL).
 * And the IntegratedPostTaskPerfTest are the same.
 * Augmented perf tests to 30 seconds which yields more reliable results.
   (and ran old ones under 30s mode too when comparing)
 * Results :
     https://docs.google.com/spreadsheets/d/100wYvbCI_dJ7gRnQiSsYaTb5OJnbF_muL6LyQWJLXSU/edit

Bug: 860252
Change-Id: I22de2409d52414524cc125b0e2fe08e2c516fcbe
Reviewed-on: https://chromium-review.googlesource.com/1127262
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574048}