[Locked Fullscreen] Disable additional keyboard shortcuts in locked fullscreen

The find bar and zoom commands in locked fullscreen mode expose an escape hatch
that can be used to exit the mode so we're disabling them.

Bug: 904637
Change-Id: I367635fde3bbcc5aa74aecc9cc25d5d39ca041c7
Reviewed-on: https://chromium-review.googlesource.com/c/1375733
Commit-Queue: Ivan Ĺ andrk <isandrk@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616662}
3 files changed