Fix buffering state for live streams

We can't buffer past the live edge, so we must account for the live
edge in buffering state calculations.

Closes #636

Change-Id: I099e68291786a462dfb63471a2eb9974643e75bd

lib/media/playhead.js

@@ -228,7 +228,12 @@ shaka.media.Playhead.prototype.onWatchdogTimer_ = function() {
   var fudgeFactor = shaka.media.Playhead.FUDGE_FACTOR_;
   var threshold = shaka.media.Playhead.UNDERFLOW_THRESHOLD_;
 
-  var duration = this.video_.duration - fudgeFactor;
+  var duration;
+  if (this.timeline_.isLive()) {
+    duration = this.timeline_.getSegmentAvailabilityEnd() - fudgeFactor;
+  } else {
+    duration = this.video_.duration - fudgeFactor;
+  }
 
   var atEnd = (bufferEnd >= duration) || (this.video_.ended);
   if (!this.buffering_) {