fix google-map _updateMarkers to set markers property correctly

Author: brandonmull

google-map.html

@@ -557,7 +557,7 @@
 
           this._observeMarkers();
 
-          this.markers = this._setMarkers(newMarkers);
+          this._setMarkers(newMarkers);
 
           // Set the map on each marker and zoom viewport to ensure they're in view.
           this._attachChildrenToMap(this.markers);