DevTools: simplify & fix property overload logic

Source/devtools/front_end/sdk/CSSStyleModel.js

Index: Source/devtools/front_end/sdk/CSSStyleModel.js
diff --git a/Source/devtools/front_end/sdk/CSSStyleModel.js b/Source/devtools/front_end/sdk/CSSStyleModel.js
index feace02c7cd1e8782800422e7950cf768ec4659e..03d6adc47f34461ab21f9a8cfc2bcb3ce0e73cba 100644
--- a/Source/devtools/front_end/sdk/CSSStyleModel.js
+++ b/Source/devtools/front_end/sdk/CSSStyleModel.js
@@ -745,7 +745,7 @@ WebInspector.CSSStyleDeclaration.prototype = {
     /**
      * @return {!Array.<!WebInspector.CSSProperty>}
      */
-    leadingProperties: function()
+    _computeLeadingProperties: function()
     {
         /**
          * @param {!WebInspector.CSSProperty} property
@@ -777,6 +777,16 @@ WebInspector.CSSStyleDeclaration.prototype = {
     },
 
     /**
+     * @return {!Array.<!WebInspector.CSSProperty>}
+     */
+    leadingProperties: function()
+    {
+        if (!this._leadingProperties)
+            this._leadingProperties = this._computeLeadingProperties();
+        return this._leadingProperties;
+    },
+
+    /**
      * @return {!WebInspector.Target}
      */
     target: function()