Introduce a MediaSession task source

[youennf]

As per @marcoscaceres comment in #314, it might be good to use a specific task source when the spec says to queue a task.

[chrisn]

#314 uses the user interaction task source. Instead of updating each mention of "queue a task", can we add a statement in, e.g., Section 3 to say "The task source for the tasks mentioned in this specification is the user interaction task source"?