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This test now presents a consistent set of supported scale factors
invariant of the running platform, avoiding any surprises in
expectations.

BUG=298406

Change-Id: Ibcfa08f42f4e97ce7cf6244fa2a50ccab309c1c9
Reviewed-on: https://chromium-review.googlesource.com/c/1351769Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611750}