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It was possible to have repeated call to OnFocusChangedEvent() between
the time the name was written in the search box, and the actual destruction
of the automation controller, which unregisters the event handlers.

Bug: 819792
Change-Id: Ied95051f7ebb07e954b8c5d7d3cff0d3e7574daa
Reviewed-on: https://chromium-review.googlesource.com/955744
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542580}