Fix the composited cursor size

In Ash, the composited cursor size changes with the display size and
doesn't match the hardware cursor. This CL fixes that and adds a test to
make sure the cursors's size match each other.

Bug: 1220826
Change-Id: I31c0aa26f5e08af0614483703d579efd6c02c246
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2949017


Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Mitsuru Oshima (OOO back on July 12)) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898408}