Add DisplayChangeObserver to RefreshRateControllerTest

This change adds DisplayChangeObserver to RefreshRateControllerTest and
makes necessary fixes to window positioning to keep the tests passing.

Bug: b:329659301
Change-Id: Id1855678b8439b3b3782300921927dd0539ba2f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5373062


Commit-Queue: Andrew Wolfers <aswolfers@chromium.org>
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282724}