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UKM enums are used by `UKM.Entries.Recorded.ByEntryHash` and
`UKM.Entries.Dropped.ByEntryHash` when displaying a breakdown of event
counts by name.

Some events were incorrectly displayed as "obsolete" in the histogram
table here https://screenshot.googleplex.com/CRmtGDOcAwQ.png
compared to the the event definition https://source.chromium.org/chromium/chromium/src/+/master:tools/metrics/ukm/ukm.xml;l=4254;drc=ec5f934779d00e3bf817be19297f72ef5bd98e9c

This patch fixes the logic to consider an event as obsolete iff
- the event itself is marked as obsolete, or
- all the metrics under this event are marked as obsolete.

Tested locally with
chromium/src/tools/metrics/histograms$ ./merge_xml.py --output output.xml ../ukm/ukm.xml enums.xml

Bug: b/163387426
Change-Id: I06ae60edb4e025cf693ad37063a45dc3bf6edd31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347386
Commit-Queue: Yue Ru Sun <yrsun@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796858}