Once a video or audio element is loaded, its default time trait is overridden by the permanent versions. However, not all elements have their correct duration set until they are played back. Video elements get their durations from a metadata event that's triggered after play back, for example. To remedy the element's duration going to NaN in between the default trait being removed and the permanent trait being properly set, the NetStreamTimeTrait is extended to take a defaultDuration member. Video and audio elements now forward their default duration value to the trait.
