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When specified, resources are only fetched over the network if they have
an existing cache entry with revalidation headers. This serves as a
lower-bound approximation of the efficacy of a client-only wifi precache
mechanism, as it is limited to URLs that are only on the client AND in
the manifests.

BUG=679533

Review-Url: https://codereview.chromium.org/2614403003
Cr-Commit-Position: refs/heads/master@{#442999}