Quote property access in CastProxy

When CastProxy refers to a proxy object, the type is necessarily
merely "Object".  This isn't specific enough for the compiler to check
access to this.videoProxy_.paused, so we convert this to use square
brackets.

Issue shaka-project#2528

Change-Id: Ic51fa9fdd20b8ac494fea1e844a945fba319f3c8

lib/cast/cast_proxy.js

@@ -449,7 +449,7 @@ shaka.cast.CastProxy = class extends shaka.util.FakeEventTarget {
    * @private
    */
   onFirstCastStateUpdate_() {
-    const type = this.videoProxy_.paused ? 'pause' : 'play';
+    const type = this.videoProxy_['paused'] ? 'pause' : 'play';
     const fakeEvent = new shaka.util.FakeEvent(type);
     this.videoEventTarget_.dispatchEvent(fakeEvent);
   }