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Note that the tests currently cover only one metric,
ReaderModeReceivedDistillability. The other still needs to have a test
added in a follow-up CL.

See go/chrome-desktop-reader-mode-dd#heading=h.5rfn5g2dude for the UKM
collection review.

Bug: 992074
Change-Id: I30b31bca6d7d3e3310c984022118a56d3b0a68b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881497Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#748788}