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This patch adds a UKM metric that tracks two boolean values recorded
once per (main frame's) document:
- Whether find-in-page happened on that page
- Whether a find-in-page activated a render-subtree element.

R=chrishtr@chromium.org, schenney@chromium.org

Bug: 1044299
Change-Id: I753702069fdad5d3145f7afe822ca638304470b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013744
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735061}