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In order to properly clip ARC notifications, we need to properly bound them in
the parent Widget layer and draw a custom border with a Z-order above all other
layers.

BUG=976952, 919642

Change-Id: I2c41eaef27798cc1009d4d4f4449674860b5dd84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877452Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709570}