Add a callback to sync popcorn initialization after video element is ready.

Due to the extra delays added by setting up the closed captions, it happens more
often that the writing.js code will attempt to initialize popcorn on the video
element before the video element actually exists on the page.

(This was always a problem, it's just worse now)

Add a synchronization point which will delay the popcorn initialization until the
video element is ready.