perf(ngStyle): use newStyles to reduce the number of element.css

Author: itchyny

src/ng/directive/ngStyle.js

@@ -54,9 +54,12 @@
 var ngStyleDirective = ngDirective(function(scope, element, attr) {
   scope.$watchCollection(attr.ngStyle, function ngStyleWatchAction(newStyles, oldStyles) {
     if (oldStyles && (newStyles !== oldStyles)) {
+      if (!newStyles) {
+        newStyles = {};
+      }
       forEach(oldStyles, function(val, style) {
-        if (!(newStyles && newStyles[style])) {
-          element.css(style, '');
+        if (newStyles[style] == null) {
+          newStyles[style] = '';
         }
       });
     }