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By spamming it with DCHECK_CURRENTLY_ON() everywhere. Since no DCHECKs
failed, delete the lock and all the related code that assume the class
can be accessed on other threads. Add threading checks to ZoomController
as well.

Change-Id: I04f8315eaebd5839889a771fd5c13bf43c4d8f31
Reviewed-on: https://chromium-review.googlesource.com/762529Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517891}