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This CL adds a new MessagePumpForUI::Observer class, which can be used to get
notifications pre and post MSG dispatch.

Change-Id: I16b0df6d30b41f04f124d4b64d850c278f2c10d9
Reviewed-on: https://chromium-review.googlesource.com/1166164
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582324}