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This is reland of crrebu.com/c/990203 with ASH_EXPORT

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.

TBR=mukai@chromium.org
BUG=b/33693796
TEST=covered by unittest

Reviewed-on: https://chromium-review.googlesource.com/990203Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#550278}
Change-Id: Ib60a03017c724c62d5912a472fa6e32dd9a02e99
Reviewed-on: https://chromium-review.googlesource.com/1011323Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550443}