Fix misc PIP multi-display bugs and refactor tests.

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}
7 files changed