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Some classes listen to ShellObserver::OnVirtualKeyboardStateChanged in
order to add/remove KeyboardController observer when the keyboard is
enabled/disabled. However, with the simplified KeyboardController
lifetimes, we can add/remove observers whenever.

So we just add an observer in the constructor and remove it in the
destructor.

After this change, there will be no more observers of
OnVirtualKeyboardStateChanged.

Bug: 731537
Change-Id: I7274484229ff5f64ac973806e565f178a271094f
Reviewed-on: https://chromium-review.googlesource.com/1101598Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567918}