Add AccessibilityPrivate methods to communicate Select-to-Speak extension state.

This is part of go/chromeos-sts-on-screen-ui, which requires the chrome process
to be able to communicate bi-directionally with the Select-to-Speak extension:
Chrome must be able to request Select-to-Speak to change states, and must get
notified whenever Select-to-Speak's state changes.

1. This change hooks up the extension receiving requests from Chrome to change
state, although the implementation of changing state logic in the extension is
currently a TODO.

2. This change also hooks up the extension communicating its state back to
Chrome, although the accessibility_extension_api method that receives the call
is implemented as a TODO after converting accessibility_private state into a
more universal mojom::SelectToSpeakState state.

Note: We need mojom::SelectToSpeakState because chrome needs to communicate
state changes through to the shelf in Ash, so the enum needs to be usable across
processes.

The next changes will complete the TODOs in 1. and 2. above.

Bug: 753018
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia1785a85adea02b3e60ed2e60525012d166e7ad2
Reviewed-on: https://chromium-review.googlesource.com/1030901
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555147}

• ash/public/interfaces/accessibility_controller.mojom[diff]
• chrome/browser/accessibility/accessibility_extension_api.cc[diff]
• chrome/browser/accessibility/accessibility_extension_api.h[diff]
• chrome/browser/resources/chromeos/select_to_speak/select_to_speak.js[diff]
• chrome/browser/resources/chromeos/select_to_speak/test_support.js[diff]
• chrome/common/extensions/api/accessibility_private.json[diff]
• extensions/browser/extension_event_histogram_value.h[diff]
• extensions/browser/extension_function_histogram_value.h[diff]
• third_party/closure_compiler/externs/accessibility_private.js[diff]
• tools/metrics/histograms/enums.xml[diff]