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This CL adds two new ResultsSeen histograms that apply to all users.

One records whether results were seen for the tap gesture, and the other
for any gesture that triggers the Contextual Search UI.

All the previous histograms slice the data one way or another, usually
based on whether the user opted-in to our privacy policy.

These new histograms will make it easy to see the overall impact of
experiments that affect CTR.

BUG=821893

Change-Id: I0a20ca56668d2086a213fd946b0704e4db601714
Reviewed-on: https://chromium-review.googlesource.com/963291Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543715}