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See https://docs.google.com/document/d/1rskVuhK9wAqgntcq9WYPSknMEc_jsAp3zrqb4YSMAqE/edit#
for the mock and expected behaviors.

See crbug.com/997885#c5 for demo. For now with this CL in place, we can
only drag the window around. More will be done in follow up Cls.

Bug: 997885
Change-Id: Iecea3a8cb4a6fe3c6a64ddddea08bc010cfb72dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817105
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700306}