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Responses served from the cache follow a very similar code path to
responses from the server, but we probably don't want to treat them the
same in metrics, so this CL breaks the metric in two.

Change-Id: I80fc9ebbfa8afc39237dc57727f13f5be68cabcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438889Reviewed-by: Bettina Dea <bdea@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812895}