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Adds a new UKM event, PrefetchProxy, which is recorded once on every
page load for Data Saver users. The metrics in the event are capped and
bucketed as described in the privacy doc (see bug).

Bug: 995437
Change-Id: Iddf5d148ccbfe784316ae57bcf439db0cdaffc85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872986
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709548}