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If there are no focusable views in the LockContentsView a crash will occur when
toggling focus. Although this doesn't currently cause a crash, it was a problem
in the past and we need to prevent it from happening in case of UI changes in
the future.

TEST=unit test
BUG=914103

Change-Id: Iee5f23d98aa202998ccd91c743e1f0279c162c81
Reviewed-on: https://chromium-review.googlesource.com/c/1493446Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636541}