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The base/derived class stuff was hiding a bug where the test wasn't
testing what it was supposed to be testing because the FeatureList was
being init'd too late.

This CL cleans all that up and adds a bunch of comments for why, while
making everything a bit easier to understand.

Bug: 1023485
Change-Id: I31b0cab43af4a3032073c69200702b004f8178b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2126469
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754738}