The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2d2df4debd2c39a974a00e6a6357586f31efba42

commit 2d2df4debd2c39a974a00e6a6357586f31efba42
Author: John Delaney <johnidel@chromium.org>
Date: Fri Sep 21 05:35:35 2018

Report ad resource sizes by mime type

Record the size of ad resources according to MIME type. This will help us
understand which resources are using up more network bytes and allow
us to better estimate the effect any ad intervention will have. Only
HTML/JS/CSS/Image/Video are recorded as they are expected to capture most
of the resources commonly seen in ads. It is expected that these metrics
will not be completely accurate because requests do not always contain
the correct content type. It is assumed that the MIME type in the response
header is a suitable proxy resource type.

Bug:  881049 
Change-Id: I000466fb3fe5467c4f73527cb4cedbd7709d7c3f
Reviewed-on: https://chromium-review.googlesource.com/1219900
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593087}
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.h
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer_browsertest.cc
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/common/page_load_metrics/page_load_metrics.mojom
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/renderer/page_load_metrics/page_resource_data_use.cc
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/renderer/page_load_metrics/page_resource_data_use.h
[add] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/test/data/ad_tagging/bear-320x240-video-only.webm
[add] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/test/data/ad_tagging/multiple_mimes.html
[add] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/test/data/ad_tagging/pixel.png
[add] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/test/data/ad_tagging/style.css
[add] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/chrome/test/data/ad_tagging/test.pdf
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/net/test/embedded_test_server/request_handler_util.cc
[modify] https://crrev.com/2d2df4debd2c39a974a00e6a6357586f31efba42/tools/metrics/histograms/histograms.xml