Bug 844275 - Update test_acceleration to guard against windows with no layer manager. r=joedrew

gfx/tests/mochitest/test_acceleration.html

@@ -33,8 +33,13 @@ var acceleratedWindows = 0;
 while (windows.hasMoreElements()) {
   windowutils = windows.getNext().QueryInterface(Ci.nsIInterfaceRequestor)
                                  .getInterface(Ci.nsIDOMWindowUtils);
-  if (windowutils.layerManagerType != "Basic") {
-    acceleratedWindows++;
+  try {
+    if (windowutils.layerManagerType != "Basic") {
+      acceleratedWindows++;
+    }
+  } catch (e) {
+    // The window may not have a layer manager, in which case we get an error.
+    // Don't count it as an accelerated window.
   }
 }