Change bufferedAhead value calculation when switching periods.

Our new logic calculating bufferedAhead value doesn't work when
it comes to switching periods for content with SegmentTemplates.
This change partially reorders code from the 12c07efc change.

Closes #545
Closes #537

Change-Id: I1d9c3ab8a185d0a3f419bbc442b366e697115dd2

lib/media/streaming_engine.js

@@ -841,15 +841,24 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
   // Compute how far we've buffered ahead of the playhead.
   var playheadTime = this.playhead_.getTime();
 
+  var reference;
+
   // Get the next timestamp we need.
+  // TODO: see if we can refactor this logic to be less cumbersome
   var bufferEnd = this.mediaSourceEngine_.bufferEnd(mediaState.type);
   var timeNeeded = this.getTimeNeeded_(mediaState, playheadTime);
   shaka.log.v2(logPrefix, 'timeNeeded=' + timeNeeded);
 
+  var currentPeriodIndex = this.findPeriodContainingStream_(mediaState.stream);
   var needPeriodIndex = this.findPeriodContainingTime_(timeNeeded);
   var needPeriod = this.manifest_.periods[needPeriodIndex];
-  var reference = this.getSegmentReferenceNeeded_(
-      mediaState, playheadTime, bufferEnd, needPeriodIndex);
+
+  if (currentPeriodIndex != needPeriodIndex) {
+    reference = null;
+  } else {
+    reference = this.getSegmentReferenceNeeded_(
+        mediaState, playheadTime, bufferEnd, needPeriodIndex);
+  }
 
   var bufferedAhead;
   if (reference) {
@@ -893,8 +902,6 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
   }
   mediaState.endOfStream = false;
 
-  var currentPeriodIndex = this.findPeriodContainingStream_(mediaState.stream);
-
   // Check if we've buffered to the end of the Period. This should be done
   // before checking segment availability because the new Period may become
   // available once it's switched to. Note that we don't use the non-existence