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This both helps organize the members of UkmRecorderImpl a bit, and
also simplifies the code for Purge(), so that it doesn't need to know
about all of the different fields. This should help make it harder to
accidently add a new field and not delete it during a Purge().

Change-Id: I49558468cfb491c34ef76d2c2801aae72ddc8364
Reviewed-on: https://chromium-review.googlesource.com/1054142
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573526}