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The CL adds media recording indicator to UserChooserView. Microphone and
camera can be used by another user when multi user login is enabled.
The indicator shows that they are in use by the background user.

Screenshot: http://screen/drwebysXfDm

TEST=manual
BUG=845806

Change-Id: I02f8912d520315f1509e9f27ec818a0eeaccbe0e
Reviewed-on: https://chromium-review.googlesource.com/1181001Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585080}