Bug 1533909 - part2 : add assertion to ensure we should always get a valid TextTrackCueList. r=jya

In this comparison, we only process `hidden` or `showing` track which should not return null TextTrackCueList.

Differential Revision: https://phabricator.services.mozilla.com/D23087

dom/html/TextTrackManager.cpp

@@ -534,6 +534,7 @@ class CompareSimpleTextTrackEvents {
       }
 
       TextTrackCueList* cueList = t1->GetCues();
+      MOZ_ASSERT(cueList);
       nsTArray<RefPtr<TextTrackCue>>& cues = cueList->GetCuesArray();
       auto index1 = cues.IndexOf(c1);
       auto index2 = cues.IndexOf(c2);