As of now it's ~12-13% and the original CL was held back, due to a data
volume concern
https://www.chromestatus.com/metrics/feature/timeline/popularity/456

As recommended in the UKM-users thread, we'll try to do 1/10 sampling
to reduce the volume.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79ac88904a1708997e2f8046d5c0c3e679b04060

commit 79ac88904a1708997e2f8046d5c0c3e679b04060
Author: Takayoshi Kochi <kochi@chromium.org>
Date: Wed May 09 04:48:48 2018

Add kElementCreateShadowRoot to UKM

Use UKM for Element.createShadowRoot() to help investigate
which site is actively using Shadow DOM V0 API.  The motivation
is to drive the usage down, by outreaching those users.

As of today the usage is around 8% so we sample down by a
factor of 10 to reduce the data volume.

Bug:  806671 
Change-Id: I9b183d7724c51979421fdfcb1ee9f19587a1f5ab
Reviewed-on: https://chromium-review.googlesource.com/890144
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557095}
[modify] https://crrev.com/79ac88904a1708997e2f8046d5c0c3e679b04060/chrome/browser/page_load_metrics/observers/use_counter/ukm_features.cc
[modify] https://crrev.com/79ac88904a1708997e2f8046d5c0c3e679b04060/chrome/browser/page_load_metrics/observers/use_counter_page_load_metrics_observer.cc