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This CL introduces a new shell type for ARC++ toasts in remote-shell.
Since the purpose of toasts is a notification for users and the toast
appears and disappears quickly without any user interaction, so toasts
are supposed to be shown over Chrome OS's virtual keyboard.

ARC++ side CL: ag/11272597
design overview: go/vk-overlays-toasts

BUG=b:109717200
TEST=ToastSurfaceTest

Change-Id: I3cec22a031a7046ff63770ff308b6d0695c313e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2167792
Commit-Queue: Toshiki Kikuchi <toshikikikuchi@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786617}