GitLab

What:
New UseCounter to tracker % of page loads that have an ad.

Why:
This metric is useful for knowing how many pageloads are applicable
for ad interventions, such as the heavy ad intervention.

Bug: 1139434
Change-Id: I8e6e1b9b6aa60afcd4d58c331c00a3b9bcf6523e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480967
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825048}