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This is a reland of d35f5dac with the addition of ensuring initialization of one of the SurfaceTracker fields in its constructor.
The original change was reverted in d35cd337 due to breakage of build Linux ChromiumOS MSan Tests since https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Tests/20302

Original change's description:
> Modify Privacy Budget UKM Entry Filter to measure identifiable surfaces.
>
> 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/+/2363398
> Reviewed-by: Asanka Herath <asanka@chromium.org>
> Commit-Queue: Russ Hamilton <behamilton@google.com>
> Cr-Commit-Position: refs/heads/master@{#800332}

Bug: 1112787
Change-Id: Iae86f07cb148ff17d449eb9ebf49b8c431391413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368461Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/master@{#800593}