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Record metrics for
* total payload size in kB,
* total payload transfer rate in kB/s
* the number of file/text attachments per payload

We also slice the first two metrics by
* send/receive
* phone/Chromebook/tablet/unknown
* success/failure/cancelled

Manually tested and verified results at chrome://histograms.

Bug: 1105579
Change-Id: I94d6283157ca65bc4cd33fcf9d0c2074fb89db66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531899Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827064}