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We have to set insets to ARC++ notification mask so that visible rect
should have appropriate rounded corners during scroll.
Ideally this can be fixed by changing layer ordering of NativeViewHost,
but it's not going to happen soon.

TEST=manual
BUG=866777

Change-Id: Ic4a2dc3c7f56051937dd02ae34fa0ab3bb290c08
Reviewed-on: https://chromium-review.googlesource.com/1215507Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590216}