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This is a reland of ebe80ad0

Original change's description:
> Wait for ukm entry in TryAutoplay
> 
> The test fails due to a race of UkmRecorder::AddEntry with DOMMessageQueue::Observe:
> https://cs.chromium.org/chromium/src/services/metrics/public/cpp/delegating_ukm_recorder.cc?q=src/services/metrics/public/cpp/delegating_ukm_recorder.cc&sq=package:chromium&g=0&l=137
> 
> Bug: 880217
> Change-Id: Icf099c3eb1094d8a571dc6f87b152e3c838807dc
> Reviewed-on: https://chromium-review.googlesource.com/1204011
> Reviewed-by: Tommi <tommi@chromium.org>
> Reviewed-by: Steven Holte <holte@chromium.org>
> Commit-Queue: Steven Holte <holte@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#589317}

Bug: 880217
Change-Id: I6c5cd4107f74f3102b6bd491323a6ab68f56f911
Reviewed-on: https://chromium-review.googlesource.com/1212802Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589499}