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The CL:
* Introduces keyboard.mojom.KeyboardEnableState, an enum used to track
  the various states that control whether the virtual keyboard should
  be enabled. Some states enable the keyboard, others disable it.
* Moves ownership of the states and the state machine to determine when
  the keyboard should be enabled to KeyboardController.
* Adds mojo interfaces to set and clear states, get the current enabled
  state, and explicitly reload the keyboard.

Note: All mojo API calls to change an enable state will also enable or
disable the keyboard accordingly. This is consistent with existing
logic where these calls are made so should not change any behavior.

Note: Not all calls to keyboard_util.cc helpers have been replaced,
including a few in src/chrome, only places where no additional
re-factoring is required were changed.

Bug: 843332
Change-Id: Ic1741188e2e1ccafda4ad95a71063462ac7f52ff

From original CL:
TBR=tsepez@chromium.org,shend@chromium.org

Change-Id: Ic1741188e2e1ccafda4ad95a71063462ac7f52ff
Reviewed-on: https://chromium-review.googlesource.com/c/1291350Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601253}