Re-do the recent performance fix

180cab1 removed unnecessary access to the cssText property when there is no onChange callback provided. However, 1f5940b undid that gain by saving the original value for later comparison.

lib/CSSStyleDeclaration.js

@@ -74,7 +74,12 @@ CSSStyleDeclaration.prototype = {
       this.removeProperty(name);
       return;
     }
-    var originalText = this.cssText;
+
+    var originalText;
+    if (this._onChange) {
+      originalText = this.cssText;
+    }
+
     if (this._values[name]) {
       // Property already exist. Overwrite it.
       var index = Array.prototype.indexOf.call(this, name);