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When a rotation is set, a very tiny error is introduced in the root
window transform due to use of trig functions in computing the transform.

This small error is later turned into a full pixel error in the root
window size.

This bug was just fixed for chromecast but also exists in TestScreen.

Use a common fix for both chromecast and TestScreen, and add a unit test
that ensures root window dimensions precisely match the display size when
a display rotation in set in the root as part of the root window transform.

Bug: 1019015

Change-Id: I930d09d5d079640b17040a721f4334c9cda0272a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1088126
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711374}