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This change resolves the UKM recording and ties it just to the
mainframe decision and result. It also flushes when the web contents
observer destructs. A future change will update the event on
rebuffers and consider more events for subframes.

Bug: 1109068
Change-Id: Iacd04920811d96c4eedd3290309a8bad93777b58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321520
Auto-Submit: Michael Crouse <mcrouse@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792513}