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This change fixes some errors in conversions between Screen and root
window coordinates. It also makes the PIP window layout tests run for
multiple display configurations.

Test: unit test
Bug: b/124689893
Change-Id: I764c48d951e36c2b4bcf1d4334bf0b36a4966e64
Reviewed-on: https://chromium-review.googlesource.com/c/1477463
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636336}