keep the most recent video frame for presentation at video end or pause
ClearFutureFrames() is not required in MaybeResolveEndPromise() when the
VideoQueue() is empty, because the queue is now not emptied until end of
playback. An empty queue before end of playback means no frames have been set
in the VideoFrameContainer.
Differential Revision: https://phabricator.services.mozilla.com/D223694
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1919683
gecko-commit: 899312c8f8609f09dc132d7d4695ff7f95aa38ad
gecko-reviewers: media-playback-reviewers, padenot