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ChromeKeyboardUI does a lot of things. This is just some small cleanup
in preparation for separating the WebContents Delegate/Observer code
from the aura::Window code.

It also adds a very basic unit test (a unit test file exists but it
contains no tests).

Bug: 843332
Change-Id: I82096babb7d8e6c95ff2033c40841392893cfa0f
Reviewed-on: https://chromium-review.googlesource.com/1182001Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584590}