I think this is indeed worthwhile; without an understanding of how things _are_ used inside extensions, it's difficult to justify decisions about how things _should_ work. For instance, https://www.chromestatus.com/metrics/feature/timeline/popularity/662 tells me that I'm going to have a hard time enforcing mixed content checks inside extensions, and is something I'm keeping my eye on for the future. We're flying blind without that data.

Luna, I think what we should do here is add at least one new histogram - Blink.UseCounter.Extensions.Features which we switch to using instead of Blink.UseCounter.Features in UseCounter::didCommitLoad if the URL is a chrome-extension:// URL (instead of disabling reporting completely).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fc32d83f891bf73c8ce299d6813baf812907a8c8

commit fc32d83f891bf73c8ce299d6813baf812907a8c8
Author: lunalu <lunalu@chromium.org>
Date: Sat May 20 04:25:28 2017

Adding UseCounter specific for extensions

Replaced Blink.UseCounter.Features by Blink.UseCounter.Extensions.Features when
URL protocol is chrome-extension://

BUG= 687169 

Review-Url: https://codereview.chromium.org/2796283005
Cr-Commit-Position: refs/heads/master@{#473409}

[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/base/test/histogram_tester.cc
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/base/test/histogram_tester.h
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/third_party/WebKit/Source/core/frame/UseCounter.cpp
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/third_party/WebKit/Source/core/frame/UseCounterTest.cpp
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/third_party/WebKit/Source/platform/testing/HistogramTester.cpp
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/third_party/WebKit/Source/platform/testing/HistogramTester.h
[modify] https://crrev.com/fc32d83f891bf73c8ce299d6813baf812907a8c8/tools/metrics/histograms/histograms.xml