Improve type safety of scroll method

Makes closure compiler happy.

Author: rictic

iron-scroll-target-behavior.html

@@ -190,18 +190,22 @@
      * Scrolls the content to a particular place.
      *
      * @method scroll
-     * @param {number|!ScrollToOptions} leftOrOptions The left position or scroll options
+     * @param {number|!{left: number, top: number}} leftOrOptions The left position or scroll options
      * @param {number=} top The top position
      * @return {void}
      */
     scroll: function(leftOrOptions, top) {
-      var left = leftOrOptions;
+      var left;
 
       if (typeof leftOrOptions === 'object') {
         left = leftOrOptions.left;
         top = leftOrOptions.top;
+      } else {
+        left = leftOrOptions;
       }
 
+      left = left || 0;
+      top = top || 0;
       if (this.scrollTarget === this._doc) {
         window.scrollTo(left, top);
       } else if (this._isValidScrollTarget()) {