Use square brackets for receiver app on window

The receiver app is attached to the window so it can be debugged.
This should use square brackets, since "receiver" is not defined on
"window".

Issue #2528

Change-Id: I8e725bb8292cdcb88ffccd3bc65d6d54181affa6

demo/cast_receiver/receiver_app.js

@@ -140,7 +140,10 @@ class ShakaReceiverApp {
 ShakaReceiverApp.IDLE_TIMEOUT_MINUTES_ = 5;
 
 document.addEventListener('shaka-ui-loaded', () => {
-  // Initialize the receiver app by instantiating ShakaReceiverApp.
-  window.receiver = new ShakaReceiverApp();
-  window.receiver.init();
+  // Instantiate ShakaReceiverApp.
+  const receiver = new ShakaReceiverApp();
+  // Attach it to window so that it can be seen in a debugger.
+  window['receiver'] = receiver;
+  // Initialize the app.
+  receiver.init();
 });