The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ec69818af043485daccd4d4f43718390e9ddcb08

commit ec69818af043485daccd4d4f43718390e9ddcb08
Author: bmcquade <bmcquade@chromium.org>
Date: Wed Feb 22 13:58:11 2017

Add metrics for tracking subresource filter activation suppression.

This change adds an API on ContentSubresourceFilterDriverFactory that
the SubresourceFilterMetricsObserver queries to determine whether
and why activation was suppressed for the currently committed page load.
The SubresourceFilterMetricsObserver then logs histograms that allow us
to understand how often activation is suppressed.

BUG= 694036 

Review-Url: https://codereview.chromium.org/2703093002
Cr-Commit-Position: refs/heads/master@{#452027}

[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/chrome/browser/page_load_metrics/observers/subresource_filter_metrics_observer.cc
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/chrome/browser/page_load_metrics/observers/subresource_filter_metrics_observer.h
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/chrome/browser/subresource_filter/subresource_filter_browsertest.cc
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/components/subresource_filter/content/browser/content_subresource_filter_driver_factory.cc
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/components/subresource_filter/content/browser/content_subresource_filter_driver_factory.h
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/components/subresource_filter/content/browser/content_subresource_filter_driver_factory_unittest.cc
[modify] https://crrev.com/ec69818af043485daccd4d4f43718390e9ddcb08/tools/metrics/histograms/histograms.xml