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This is an interim step to handle FeaturePolicy-enabled features, before the
three conditional feature flags (RuntimeEnabled, OriginTrialEnabled and
FeaturePolicy) are eventually combined into a common 'Feature' attribute.

BUG=638240

Review-Url: https://codereview.chromium.org/2247923004
Cr-Commit-Position: refs/heads/master@{#412524}