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1. This allows other constraints about prefetched resources to be applied.
2. Together with another CL https://codereview.chromium.org/793823002/, the prefetched resources can temporarily bypass revalidation.

BUG=405690, 406209

Review URL: https://codereview.chromium.org/891343007

Cr-Commit-Position: refs/heads/master@{#315369}