Fix weird page-load race on IE 11

Sometimes, on IE 11, the demo app fails to load because "shaka" is
undefined when shakaDemo.init() is called.  This seems to have been
introduced in 5126324 when we started deferring script loading.
This bug has not appeared in a release version yet.

It seems that IE 11 sets the readyState on the document at the wrong
time when scripts are deferred.  Edge and Chrome both do the right
thing.

This came up during debugging while working on #1111, but this fix is
not part of the fix for #1111.

Change-Id: Ia42466acdced7ca67fed5a6e8270620b7d8dcd50

demo/main.js

@@ -663,5 +663,17 @@ if (document.readyState == 'loading' ||
     window.addEventListener('load', shakaDemo.init);
   }
 } else {
-  shakaDemo.init();
+  /**
+   * Poll for Shaka Player on window.  On IE 11, the document is "ready", but
+   * there are still deferred scripts being loaded.  This does not occur on
+   * Chrome or Edge, which set the document's state at the correct time.
+   */
+  var pollForShakaPlayer = function() {
+    if (window.shaka) {
+      shakaDemo.init();
+    } else {
+      setTimeout(pollForShakaPlayer, 100);
+    }
+  };
+  pollForShakaPlayer();
 }