[EME] License request generation and session loading moved to MediaKeySession

See https://www.w3.org/Bugs/Public/show_bug.cgi?id=26575#c10. Let me know
if I missed any related changes.

I didn't make createSession() synchronous yet, but I intend to unless we
hear reasons that it should remain asynchronous. The algorithm
<https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/encrypted-media.html#dom-createsession>
is now very simple.

David