pause() should return a Promise, allowing currentTime to progress slightly

[foolip]

The way that pause() is defined requires that after pausing, currentTime remains the same as when pause() was invoked. The final steps should wait for the media framework to actually pause, and then use the final currentTime.

It could be spec'd such that the timeupdate event is guaranteed to only fire once after pause() is invoked and that this would be the final currentTime, but that seems a bit tricky if there's already a pending timeupdate event. Instead I would like for pause() to return a promise.

CC @rocallahan @jernoble @eric-carlson

[foolip]

This was originally reported in https://www.w3.org/Bugs/Public/show_bug.cgi?id=28125