Add a ScopedFeatureList class for testing and start using it.

This class improves upon the current situation by ensuring a test
doesn't end up modifying the global feature list permanently.

Migrates a number of tests to new the scoped class.

BUG=620435
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Review-Url: https://codereview.chromium.org/2226063002
Cr-Commit-Position: refs/heads/master@{#410589}
30 files changed