Fix test for bug 474056 to run on non-Windows 7 machines.

--HG--
extra : rebase_source : 8d1c5f3f8dd4e9df22c60a0694b712454b7e7077

browser/components/wintaskbar/test/browser_taskbar_preview.js

@@ -2,10 +2,10 @@ function test() {
   waitForExplicitFinish();
 
   // Cannot do anything if the taskbar service is not available
-  ok(AeroPeek.available == isWin7OrHigher(), "AeroPeek initialized when it should be");
+  ok(("AeroPeek" in window) == isWin7OrHigher(), "AeroPeek initialized when it should be");
 
   // Entire feature is disabled
-  if (!AeroPeek.available)
+  if (!("AeroPeek" in window))
     return;
 
   gPrefService.setBoolPref("aeropeek.enable", true);