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This is a reland of b03ffb6b

Ran arc.Boot locally and it passes.

TBR=jamescook@chromium.org

Original change's description:
> [VK] Clarify KeyboardUI interface.
>
> Currently, KeyboardUI has a weird interface where |GetKeyboardWindow|
> will start loading the keyboard web page if it is called the first time.
> So this resulted in a lot of places where we were calling
> GetKeyboardWindow and didn't know whether it would cause a load or not.
>
> We change the KeyboardUI interface a bit to explicitly state when we
> are creating / loading a keyboard window, vs just getting an already
> loaded one.
>
> This removes ChromeKeyboardUIWebContent's dependency on
> KeyboardController as well.
>
> We tried to keep the same behaviour for existing KeyboardUI subclasses.
>
> Bug: 845780
> Change-Id: I3178c81c382f2c3cd8217eda5a0c9b9d489df5be
> Reviewed-on: https://chromium-review.googlesource.com/c/1264336
> Commit-Queue: Darren Shen <shend@chromium.org>
> Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#599772}

Bug: 845780
Change-Id: I77587e5bb62592df6d7cc9497e370600126c9418
Reviewed-on: https://chromium-review.googlesource.com/c/1288175Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601043}