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This will be used in an upcoming CL. We want the Picture-in-Picture
window to avoid system ui areas such as the message center and virtual
keyboard. In order to support this, this CL adds a new WM event to fire
when something changes about a 'system ui area,' which currently
includes the virtual keyboard (floating or normal) and the message
center.

Bug: b/115291749
Bug: 841886
Bug: 874545
Bug: b/112668491
Test: Added unittest
Change-Id: Ic3b0cde8800bfc98dd2ffbe0d17137a6e1a26809
Reviewed-on: https://chromium-review.googlesource.com/c/1219366
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601832}