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This will be used for ARC++ windows. This
a) is always placed at the topof the display and covered full width.
b) supports immersive mode
c) is resized when the display metrics changes.

BUG=b/33693796
TEST=covered by unittest

Change-Id: I27c8e9d27b673b5f85ff6992380c8ea707d3ea55
Reviewed-on: https://chromium-review.googlesource.com/990203Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550278}