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The test was added to check if the panel produces expected dsf.
This CL enahance the test to check if it produces expected size in dp
and screenshot size.

Bug: None

Change-Id: I1112df16156352ed126db968da1427d257395c21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094575Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748421}