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This simplifies the interface for the Previews Decider and UI service

Change-Id: Id50c986ba9e5fe00f6d8547101b1a7d162fc9584
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925434Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716869}