fix(text): Fix VTTCue polyfill in uncompiled mode on Edge

The polyfill assigns a static method over window.VTTCue, which is then
called with "new".  This will not work on Edge in uncompiled mode,
since the method is not an old-style "function".

To fix this, we wrap the chosen static method into a "function".

Change-Id: I9b5687e39b3af1bd160e4db4595de06720564a1d

lib/polyfill/vttcue.js

@@ -28,17 +28,32 @@ shaka.polyfill.VTTCue = class {
       return;
     }
 
+    /** @type {?function(number, number, string):!TextTrackCue} */
+    let replacement = null;
     const constructorLength = TextTrackCue.length;
     if (constructorLength == 3) {
       shaka.log.info('Using VTTCue polyfill from 3 argument TextTrackCue.');
-      window['VTTCue'] = shaka.polyfill.VTTCue.from3ArgsTextTrackCue_;
+      replacement = shaka.polyfill.VTTCue.from3ArgsTextTrackCue_;
     } else if (constructorLength == 6) {
       shaka.log.info('Using VTTCue polyfill from 6 argument TextTrackCue.');
-      window['VTTCue'] = shaka.polyfill.VTTCue.from6ArgsTextTrackCue_;
+      replacement = shaka.polyfill.VTTCue.from6ArgsTextTrackCue_;
     } else if (shaka.polyfill.VTTCue.canUse3ArgsTextTrackCue_()) {
       shaka.log.info('Using VTTCue polyfill from 3 argument TextTrackCue.');
-      window['VTTCue'] = shaka.polyfill.VTTCue.from3ArgsTextTrackCue_;
+      replacement = shaka.polyfill.VTTCue.from3ArgsTextTrackCue_;
     }
+
+    if (!replacement) {
+      shaka.log.error('No recognized signature for TextTrackCue found!');
+      return;
+    }
+
+    // The polyfilled VTTCue must be callable with "new", but the static methods
+    // in this class cannot be called that way on legacy Edge.  So we must wrap
+    // the replacement in a plain function.
+    // eslint-disable-next-line no-restricted-syntax
+    window['VTTCue'] = function(start, end, text) {
+      return replacement(start, end, text);
+    };
   }
 
   /**