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There are UMA metrics to measure the duration from when the API is
called to when the SMS is received / a user clicks "Allow" to complete
the flow. We want to add corresponding UKMs to help with the feature
adoption.

Note that the UKM that records the duration from when the API is called
to when a user clicks "Deny" to abort the flow will be added in a
follow up patch after crrev.com/c/2428825.

UKM collection review:
https://docs.google.com/document/d/15orQiLx06QSuZr4ShBaozY0dvPzXiTWwJZ9Xk5cgzAA/edit?usp=sharing

Bug: 1127402
Change-Id: I204bd469ca203182eba6a37f3522d7a492cc937d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429236Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Sam Goto <goto@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814701}