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This UKM helps us to understand whether WebOTP increases phone number
collection. We will first use UMA to analyze the portion of each bucket
in the metric. If certain buckets have high volume, we need to turn to
UKM for better understanding. See detailed design doc [1].

UKM collection review:
https://docs.google.com/document/d/1xlV8thqJ3Vt_uYr0Dbi8PR_cctRzUXcDS2y1BiPGCm8/edit?usp=sharing

[1] https://docs.google.com/document/d/1OPqPNY9l76kvaBPBCS5wbBQv2HNhmNlsDAcUvmYgyNI/edit#heading=h.xlif2jhriq2t

Bug: 1107590
Change-Id: Ia6f87d35861879cfd586a20b2eb0e2baf31b9190
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359403Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799229}