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This CL adds reporting Data savings based on original content length
header vs content length header in the HTTPS Server previews URLLoader
implementation.

Bug: 921757
Change-Id: I38ac67b43b3fd1bc17f27a4ec7e0a8a65229cc81
Reviewed-on: https://chromium-review.googlesource.com/c/1488062
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635736}