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After r808115 (http://crrev.com/c/2415928) there was a corner case where
the WebContentsAccessibilityAndroid could change, but the root
BrowserAccessibilityManagerAndroid wouldn't be pointing to it.
This would manifest as TalkBack not speaking in response to the page
changing.

Bug: 1126657, 1131356
Change-Id: I99041aeae4e0fc0d643b86c3e99212dce626b9f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425292Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809806}