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This CL adds the class MessageLoopObserver, which forwards events from the UI
and IO thread message loop task runners to the responsiveness Watcher and
Calculator.

Bug: 859155
Change-Id: Ie492f1fbc26940007cb303bdc5cdbdb59d887c19
Reviewed-on: https://chromium-review.googlesource.com/1149042
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579083}