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This includes:
* keyboard_end_to_end_browsertest.cc
* keyboard_controller_browsertest.cc

Note: This renames the tests in keyboard_controller_browsertest from
VirtualKeyboard* to KeyboardController* to better match the file name
and the specific area of test focus.

This also contains some minor cleanup to keyboard::KeyboardController
that primarily impacts the in-progress SingleProcessMash implementation,
but should not affect the classic Ash implementation (except to clean
it up some). It is useful to isolate these changes from the larger CL
introducing AshKeyboardUI.

Bug: 876138
Change-Id: I8aa7e5da1f04f0d8b17a58e9582608e6159bee29
Reviewed-on: https://chromium-review.googlesource.com/c/1336250
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608638}