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As opposed to a full prerender, a prefetch could be repeated relatively soon.
This CL prevent this by detecting repeated prefetches and cancelling them with
FINAL_STATUS_DUPLICATE.

The threshold used is net::HttpCache::kPrefetchReuseMins (5 minutes), which
should roughly match what is happening for full prerenders.

Review-Url: https://codereview.chromium.org/2614473002
Cr-Commit-Position: refs/heads/master@{#441397}