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This will allow a button in the shelf to request Select-to-Speak state changes
and to be updated when Select-to-Speak updates its own state.

This builds off of https://chromium-review.googlesource.com/c/chromium/src/+/1030901
and the full working proof of concept can be seen at https://chromium-review.googlesource.com/c/chromium/src/+/1031550.

Still TODO: Add the button, and update the SelectToSpeakEventHandler to start
processing touch events or mouse when the button was clicked rather than just
when the search key was held down.

This change also adds tests for toggling Select-to-Speak on and off in
accessibility_manager, as those tests were missing.

Bug: 753018
Change-Id: I93bb77dc23bdfa855e991e31d67dc4e3b8271036
Reviewed-on: https://chromium-review.googlesource.com/1036198
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555451}