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After https://crrev.com/e90092dea, the --force-prefers-reduced-motion
flag could be accidentally ignored if the system-level setting was
changed after the browser had started. This CL restores the behavior so
that the flag always wins, and adds a test to ensure that.

Bug: None
Change-Id: I46927e3821f1118f38f4896d07755b3eaebe48b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921553Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720197}