Limit DOMMatrix's special stringifier to Window

Fixes w3c#145.

Tests: web-platform-tests/wpt#5895

Author: zcorpan

geometry/Overview.bs

@@ -682,7 +682,7 @@ interface DOMMatrixReadOnly {
     [NewObject] Float32Array toFloat32Array();
     [NewObject] Float64Array toFloat64Array();
 
-    stringifier;
+    [Exposed=Window] stringifier;
     serializer = { attribute };
 };