fix(dash): Notify segments on all updates.

Before, when using SegmentTemplate or SegmentList, we would tell
the presentation timeline about new segments only when we saw a
new period or representation. This meant that, in live streams,
if new content was being added to the SegmentTemplate or SegmentList,
the presentation timeline was not being kept abreast of the changes.
Thus, if that stream were to then transition to VOD, the presentation
timeline would suddenly start to report that the current time was
beyond the end of the seek range, which caused playback to stop.

Closes shaka-project#3675

Change-Id: I77ef00c5a86de9ebcb86f18f792084ed70f94e8d

lib/dash/segment_list.js

@@ -66,9 +66,9 @@ shaka.dash.SegmentList = class {
       const start = context.presentationTimeline.getSegmentAvailabilityStart();
       segmentIndex.mergeAndEvict(references, start);
     } else {
-      context.presentationTimeline.notifySegments(references);
       segmentIndex = new shaka.media.SegmentIndex(references);
     }
+    context.presentationTimeline.notifySegments(references);
 
     if (!context.dynamic || !context.periodInfo.isLastPeriod) {
       const periodStart = context.periodInfo.start;

lib/dash/segment_template.js

@@ -120,9 +120,9 @@ shaka.dash.SegmentTemplate = class {
         segmentIndex.mergeAndEvict(references,
             context.presentationTimeline.getSegmentAvailabilityStart());
       } else {
-        context.presentationTimeline.notifySegments(references);
         segmentIndex = new shaka.media.SegmentIndex(references);
       }
+      context.presentationTimeline.notifySegments(references);
 
       if (shouldFit) {
         segmentIndex.fit(periodStart, periodEnd);