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Previously, the buttons are on the dedicate widget to show them above
the ARC notification (NativeViewHost). This CL moves them to a view.

In addition, this CL adds a hacks for ARC notification which manipulates
the hierarycy of layers to show the buttons at top most.

Bug: b/62771497
Bug: 717455
Bug: 747217
Change-Id: I876c653e01d4a4237055bd4f763aa55017ba38ef
Reviewed-on: https://chromium-review.googlesource.com/579107
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488599}