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This struct keeps all the metrics about prefetching actions. This CL is
in preparation for recording these metrics in UKM by plumbing them out
of the TabHelper.

Two small behavioral changes:
* all predicted URLs will now be checked for eligibility, even if the
  max prefetching limit has already been reached. In reality, this
  would probably never happen anyways but it came up in testing.
* Redirects count against attempts. This way, eligible and attempt #s
  always line up with respect to the maximum number of prefetches

Bug: 1064726
Change-Id: I5d0f0b9706c12952f65335e67915b04eea60ed7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125615
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754880}