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This is a reland of f672068e

The tests timeout with closing the tab right after the navigation, so
wait to close the tab until after the histograms are recorded.


Original change's description:
> [LiteVideo] Rework UKM to record on DidFinish and WCO destruction.
>
> 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}

TBR=tbansal@chromium.org

Bug: 1109068
Change-Id: I99b458c593ea1b430fe5f35cbd8af4554942a3ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325247
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Auto-Submit: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792835}