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This makes the HintsFetchedCallback more general and paves the way for
the async CanApplyOptimization where we need to associate each hints
fetching attempt with its URL.

Change-Id: I1c0e809dbcd4244ecc268ec77a107d115291bfd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013841
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733833}