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It only needs main thread scheduler to initialize,
MessageLoop was unnecessary there.

Bug: 828835
Change-Id: I70235bbd06af1ca6c576cc77e3e4722a27225069
Reviewed-on: https://chromium-review.googlesource.com/1169211Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Greg Kraynov <kraynov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582125}