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The rate limiting is based on the number of attempted prefetches in the
last minute (technically, it is the number of prefetches in the caching
limit that were not served).

Bug: 1138650
Change-Id: I0ed6108a8f065c2fb7acc5a90521d978dca0f830
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490551Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819894}