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This patch cleans up flakiness caused by video-overlay-scroll.html which
had a fullscreenchange event handler that fired twice, but was only
intended to fire once in the test. The test used waitForEventAndEnd
which continued calling the event handler after the test ended. This
patch updates this function to waitForEventOnceAndEnd which makes it
clear that the event handler will be removed after firing once.

The root-cause of this flakiness is being fixed in
https://crrev.com/c/2275348 and this patch is just followup cleanup.

Bug: 1048597
Change-Id: I4b7473879085e5a7f91875f4daa53a2891c9274e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275350Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784142}