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This is a reland of fc5589ac

A BUILD.gn dep has been added back, which was causing linker failures.

Control flow in AccessibilityBridgeBrowsertests has been updated so
tests are no longer failing.

Original change's description:
> [Fuchsia] Send Chrome a11y updates to Fuchsia SemanticsManager
>
> 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}

Bug: 973095
Change-Id: Ie6eeb74146682838efaefb0e1051d3e9442f0d86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913569Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715126}