fix(play-toggle): call event.stopPropagation in the click handler (#5803)

The play button stops working when recent versions of Google's Polymer is in use on the page because of the way Polymer synthesizes 'tap' events on non-touch devices. The Polymer tap code thinks the click event was ignored unless the DOM event's stopPropagation method is
called.

In chrome 70 the small play/pause control doesn't work with Polymer 1.x Gestures tap is used on document. Demo of issue: https://codepen.io/mscalora/pen/mQzQmp

Fixes #5624.

src/js/control-bar/play-toggle.js

@@ -61,6 +61,7 @@ class PlayToggle extends Button {
     } else {
       this.player_.pause();
     }
+    event.stopPropagation();
   }
 
   /**