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This CL implements an alert bubble to prevent fake login.

When a device returns from sleep or low brightness and the active
window is fullscreen. Others can utilize the fullscreen window
to imitate the lock/login screen to steal user's password.

To prevent this, we generate an alert bubble to remind the user
to exit fullscreen before entering the password. The users can
also use the button in the bubble to exit fullscreen.

Bug: 1107185
Change-Id: I181f1173777ada1ec45fe7148e7dc889668d6753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505335
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822564}