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For mash, the ash window manager runs outside the browser process.
Remove direct access to ash::Shell from ChromeVoxPanel.

* Init the panel widget using mus properties
* Introduce mojo API for setting whether the panel should be fullscreen
* Remove mash checks from AccessibilityManager
* Close ChromeVoxPanel immediately on shutdown and during reloads
by session state changes and multi-profile user switching. This helps
prevent having multiple panels open during shutdown, especially in
tests.

ChromeVox doesn't read things yet under mash, but at least the panel
opens now.

Bug: 628655
Test: existing ash_unittests, browser_tests
Change-Id: I14cbf946809787791115930ad33c1c1d3357bf88
Reviewed-on: https://chromium-review.googlesource.com/956235Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542513}