fix(util): check for definition of window.performance.now before using in mdUtil (#9664)

The previous behavior assumes the existence of the now() function in browsers that provide the 
window.performance API. This change verifies it is available and if not, uses existing fallbacks.

Author: justinappler

src/core/util/util.js

@@ -59,7 +59,7 @@ function UtilFactory($document, $timeout, $compile, $rootScope, $$mdAnimate, $in
 
   var $mdUtil = {
     dom: {},
-    now: window.performance ?
+    now: window.performance && window.performance.now ?
       angular.bind(window.performance, window.performance.now) : Date.now || function() {
       return new Date().getTime();
     },