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This simplifies the public KeybardController interface and helps
prepare for supporting different InputMethodKeyboardController options
in Mash.

Bug: 845780
Change-Id: I8d857ca67c8ea57de76036ae1487abd9dbbbd91a
Reviewed-on: https://chromium-review.googlesource.com/c/1337287Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609412}