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* Emit ATK focus and object:state-changed:focused events in response
  to ax::mojom::Event::kActiveDescendantChanged
* Add ATK_STATE_FOCUSED to the state set of the active descendant
* Update tests to reflect new events and states

Bug: 959857
Change-Id: Idb97671e1c4f65aa293e1e4e7dbd5a9ecf47fb2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631404Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Commit-Queue: Joanmarie Diggs <jdiggs@igalia.com>
Auto-Submit: Joanmarie Diggs <jdiggs@igalia.com>
Cr-Commit-Position: refs/heads/master@{#663771}