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This adds unit tests for scale-aware clients that disable the
default_dsf scale cancellation. These tests determine that the server
commits the scale appropriately for different windowing modes.

BUG=b:160350706
TEST=exo_unittests

Change-Id: Idaaea44d60dbeb8e3e1d805484ce1fbff1494560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399762
Commit-Queue: Prabir Pradhan <prabirmsp@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow: gardener) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805896}