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Adds a use counter for the heavy ad intervention and adds the
use counter to the UKM whitelist. This will be used to look
at % page loads affected and also whether certain origins are more
affected than others.

Bug: 985890
Change-Id: I09f0f6b8fd2703b4801b9958d55fcac165658c2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846174
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704224}