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This change adds functionality to the Accessibility Bridge:
* Listens for change in AccessibilityMode from Fuchsia
* Gets the semantic tree from Chrome accessibility and sends it to the
SemanticsManager
* Sends live updates when nodes are added, updated or removed

To send updates, this change includes the following:
* Creates a ViewRefPair and registers it with the SemanticsManager
* Initialize an AccessibilityBridge
* Listen for changes in the Chrome Accessibility tree, serializes them
to Fuchsia data structures and sends that data to the Fuchsia
SemanticsManager

Testing: Added AccessibilityBridge browsertests.

Bug: 973095
Change-Id: I89658288b846b6b9ce84a04e49434ec266b84dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787171
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714588}