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Previously, the login screen would crash in both these cases (e.g. if
the powerwash confirmation dialog popped up and the user pressed
left/right arrow).

Change-Id: I5880d72112508de93e77c507239ea28c475d86ae
Reviewed-on: https://chromium-review.googlesource.com/1174907
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583083}