The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bf408599fde14b32f3131691370975cdb8652347

commit bf408599fde14b32f3131691370975cdb8652347
Author: Takayoshi Kochi <kochi@chromium.org>
Date: Thu May 17 05:21:01 2018

Opt-in DocumentRegisterElement to UKM

Use UKM for document.registerElement() to help investigate
which site is actively using Custom Elements V0 API.
The motivation is to drive usage down, by outreaching those
users.

As of today the usage is around 5% so we sample this feature
as well as the existing kElementCreateShadowRoot.
https://www.chromestatus.com/metrics/feature/timeline/popularity/457

Bug:  843080 
Change-Id: I8a94fdcf2d0c905a3a978cb78673c0544b2155a2
Reviewed-on: https://chromium-review.googlesource.com/1059100
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559425}
[modify] https://crrev.com/bf408599fde14b32f3131691370975cdb8652347/chrome/browser/page_load_metrics/observers/use_counter/ukm_features.cc
[modify] https://crrev.com/bf408599fde14b32f3131691370975cdb8652347/chrome/browser/page_load_metrics/observers/use_counter_page_load_metrics_observer.cc