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This also seems like a good metric to have, only recorded when
prefetching attempts have been made.

Bug: 1023485
Change-Id: I34c647d20c165386221c117e18f8212882fc9db2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129891Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755159}