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The assertions were unclear because the test window was being created
partially outside the display work area, resulting in it being moved
from its initial coordinates.

Make the simulated display bigger so the window isn't moved.
Clarify and document the expectations.

Bug: 890633
Test: ash_unittests
Change-Id: Ie0ba065bc4d8f46a2cfd6ad24250d808845e8cd5
Reviewed-on: https://chromium-review.googlesource.com/1252384
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595456}