fix(text): Fix bug where cue comparison throws

The cue comparison method references "this.equal",
which worked on the initial invocation of the method
but would fail on recursive calls, as "this" was
dereferenced along the way.
This was causing the cue comparison method to fail
between cues that had nested cues.
This changes the reference to be a static reference.

Change-Id: I1b8829987e3608db449213d7aafc84604fc7edac

lib/text/cue.js

@@ -255,8 +255,11 @@ shaka.text.Cue = class {
   static equal(cue1, cue2) {
     for (const k in cue1) {
       if (k == 'nestedCues') {
+        // This uses shaka.text.Cue.equal rather than just this.equal, since
+        // otherwise recursing here will unbox the method and cause "this" to be
+        // undefined in deeper recursion.
         if (!shaka.util.ArrayUtils.equal(
-            cue1.nestedCues, cue2.nestedCues, this.equal)) {
+            cue1.nestedCues, cue2.nestedCues, shaka.text.Cue.equal)) {
           return false;
         }
       } else if (k == 'region' || k == 'cellResolution') {