Include all attributes in DOMMatrixReadOnly JSON serialization

Fixes https://lists.w3.org/Archives/Public/public-fx/2016JanMar/0014.html

Author: zcorpan

geometry/Overview.bs

@@ -474,7 +474,7 @@ interface DOMMatrixReadOnly {
     Float32Array        toFloat32Array();
     Float64Array        toFloat64Array();
                         stringifier;
-                        serializer = [ m11, m12, m13, m14, m21, m22, m23, m24, m31, m32, m33, m34, m41, m42, m43, m44 ];
+                        serializer = { attribute };
 };
 
 [Constructor(sequence<unrestricted double> numberSequence),