The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a7da0ecfa8023b3a25f81261ac919268eaa71bc

commit 5a7da0ecfa8023b3a25f81261ac919268eaa71bc
Author: Becca Hughes <beccahughes@chromium.org>
Date: Tue Jul 04 11:50:23 2017

Record UMA metrics for -webkit-media-controls-*

We are planning on refactoring the Media Controls UI code and we
will need to record UMA metrics to see how many developers are
overriding the internal -webkit-media-controls-* CSS classes to
identify which ones can be deprecated.

BUG= 738914 

Change-Id: I33107272479457c1c143626d5524faff20d75bb6
Reviewed-on: https://chromium-review.googlesource.com/558923
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484088}
[modify] https://crrev.com/5a7da0ecfa8023b3a25f81261ac919268eaa71bc/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
[modify] https://crrev.com/5a7da0ecfa8023b3a25f81261ac919268eaa71bc/third_party/WebKit/public/platform/WebFeature.h
[modify] https://crrev.com/5a7da0ecfa8023b3a25f81261ac919268eaa71bc/tools/metrics/histograms/enums.xml