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This CL is a pure refactoring. This moves the duplicated implementations
of GetEntries/GetMetrics from test files to TestUkmRecorder.

Bug: 1033410
Change-Id: I97c4686421fb26d7084414826e5dc99e778045bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986655
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732343}