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Background:
Currently page load metrics tracks data usage when requests complete.
This misses canceled requests when the tab closes. The current codepath
will also deprecated due to the upcoming network servicification work.

This CL reports the continuous data use to page load metrics in renderer.
Response starts and completions are reported as well so that the observer
can keep track of data per resource. Subsequent CLs will send this to
browser process via the existing PageLoadMetrics mojo.

Bug: 836029
Change-Id: I023ea08b2b8f1974ea40586afeee3a27b9900782
Reviewed-on: https://chromium-review.googlesource.com/1042795
Commit-Queue: rajendrant <rajendrant@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568353}