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Add TaskScheduler Support to the MessageLoop Unit Tests

With TaskScheduler redirection, the MessageLoop will redirect tasks
only if a TaskScheduler is present. Otherwise, it will handle task
dispatch itself similar to today.

Direct dispatch is only expected to happen in unit tests and some
integration tests.

BUG=749310

Change-Id: Ia35f17a7a15155c8893020e7174573144bf7afb7
Reviewed-on: https://chromium-review.googlesource.com/932827
Commit-Queue: Robert Liao <robliao@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543232}