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This also includes some cleanup of the BUILD.gn files.

Currently TestKeyboardControllerObserver is only needed by
AshKeyboardControllerUnitTests, but will be required by any tests
affecting the keyboard with SingleProcessMash once enabled.

Bug: 843332
Change-Id: I3c321cfaefe55bb61bb11f2232f524714f6453f5
Reviewed-on: https://chromium-review.googlesource.com/c/1343524
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610881}