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This cl handles the ui::VKEY_BACK. If it is already empty search box and
typing ui::VKEY_BACK will exit search mode in Keyboard Shortcut
Viewer (KSV).

Bug: 768932
Test: Tested on device.
Change-Id: I5cee02a97959784163e65e300b5113f2a3409513
Reviewed-on: https://chromium-review.googlesource.com/923090
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537457}