Bug 883731 - Remove ResourceLoaded notifications since they don't make much sense with a media cache. r=cpearce

Based on roc's patch in https://bugzilla.mozilla.org/page.cgi?id=splinter.html&bug=883731&attachment=767505.
Bug 883731 - Remove unnecessary update of readyState and introduce NEXT_FRAME_UNAVAILABLE_SEEKING so that we return HAVE_METADATA while seeking.  r=cpearce
Bug 883731 - Progress update should be associated with network state transitions. Also centralize network state transition code.  r=cpearce
Bug 883731 - Per spec. autoplay activation should be handled by readyState transition.  r=cpearce