The DOMRectReadOnly interface requires serializer.

third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html

Index: third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html
diff --git a/third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html b/third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html
index a9989a3fd15f6b7e1895a68f388482e7b4aefe24..611e2b774797af65c7648d88ac0252efd2fd7604 100644
--- a/third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html
+++ b/third_party/WebKit/LayoutTests/fast/dom/geometry-interfaces-dom-rect.html
@@ -41,6 +41,11 @@ test(() => {
 
 test(() => {
   var rect = new DOMRect(10, 20, 80, 50);
+  assert_object_equals(rect.toJSON(), {x: 10, y: 20, width: 80, height: 50, top: 20, right: 90, bottom: 70, left: 10});
+}, 'DOMRect toJSON');
+
+test(() => {
+  var rect = new DOMRect(10, 20, 80, 50);
   rect.x = 30;
   assert_dom_rect_equals(rect, [30, 20, 80, 50, 20, 110, 70, 30]);
   rect.y = -10;
@@ -84,4 +89,9 @@ test(() => {
   assert_readonly(rect, 'height');
 }, 'DOMRectReadOnly readonly test');
 
+test(() => {
+  var rect = new DOMRectReadOnly(10, 20, 80, 50);
+  assert_object_equals(rect.toJSON(), {x: 10, y: 20, width: 80, height: 50, top: 20, right: 90, bottom: 70, left: 10});
+}, 'DOMRectReadOnly toJSON');
+
 </script>