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Add a DisplayCutoutClient mojo interface that is bound to a
frame and called from the browser. This will set safe-area*
CSS env() variables that websites can use to layout their
content based on the safe area.

DisplayCutoutClient will be called from Android in a future
CL.

BUG=847652

Change-Id: I6180aa5337e182fd1794586c47caef0f19fab718
Reviewed-on: https://chromium-review.googlesource.com/1083291Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569427}