Remove spurious second caption div.

This is a version of the nsVideoFrame changes from my original patch
which were inadvertently included in the merge. They're as superceeded
by Jason Silver's patch in m-c from bug 833388.

Fixes a crash in the layout/css code whenever loading a media element.
Call LoadResource upon LoadListener instantiation.
Add a TextTrack PR_LOG module for debugging.
Prevent the segfault when adding a cue.