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The underlying accessibility code has been refactored recently and I
cannot reproduce flake locally. Also, the flakiness dashboard shows
that the other tests in the suite are not flaky anymore. I suspect
the issue is fixed.

Also add a RunLoop spin in one place that changes a pref that ash
may need to observe. I don't think that affects this test, but I
noticed it while investigating.

Bug: 787024
Test: browser_tests
Change-Id: I65175c4e87c09c2cc172174c9b0595e4372ad652
Reviewed-on: https://chromium-review.googlesource.com/889645Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532588}