GitLab

Add a "SurfaceTracker" to IdentifiabilityStudyState to keep track of
surfaces that have been previously reported for each source. This is
used for deduplication and to limit the number of metrics sent in each
UKM report.

Bug: 1112787
Change-Id: I00eb2360849ea6772fbfdcc9b1cf91680f8c6173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363398Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/master@{#800332}