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The number of bytes from cache are saved on each request and then
made into a fraction of the total bytes on the page (cache + network).
This fraction is bucketed for privacy sake and then sent in the
pingback.

Also adds connection type, which is also a per-page metric.

Bug: 830949
Change-Id: Ia17242cc9a54acf061a72f03b7f327e141d6f3c8
Reviewed-on: https://chromium-review.googlesource.com/1026522
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556065}