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Migrate the FeatureListTest suite to use ScopedFeatureList to ensure
that the process-global FeatureList instance is replaced & restored
consistently.

Change-Id: Ia9dd5bf7269ab05d00780c25c8f3154d663546e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848354Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703929}