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Filter out any event or metric in ukm.xml that has an <obsolete> tag.
Remove <obsolete> tags from in ukm.xml for metrics still being referenced.

Bug: 1055550
Change-Id: If6538ecd87ddc6a8456c40fe08edd61c77664c37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083603
Commit-Queue: Adam Trudeau-Arcaro <adamta@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746941}