[css-typed-om] styleMap should have SameObject extended attribute.

third_party/WebKit/Source/core/dom/Element.idl

Index: third_party/WebKit/Source/core/dom/Element.idl
diff --git a/third_party/WebKit/Source/core/dom/Element.idl b/third_party/WebKit/Source/core/dom/Element.idl
index c6869f2af61f21362d76336be0d83ad52dc367cc..47867a1e87d24b59fa69f1a172a5b69207cb357f 100644
--- a/third_party/WebKit/Source/core/dom/Element.idl
+++ b/third_party/WebKit/Source/core/dom/Element.idl
@@ -116,7 +116,7 @@ interface Element : Node {
 
     // Typed OM
     // https://drafts.css-houdini.org/css-typed-om/#inline-stylepropertymap-objects
-    [RuntimeEnabled=CSSTypedOM] readonly attribute StylePropertyMap styleMap;
+    [RuntimeEnabled=CSSTypedOM, SameObject] readonly attribute StylePropertyMap styleMap;
 
     // Non-standard API
     [MeasureAs=ElementScrollIntoViewIfNeeded] void scrollIntoViewIfNeeded(optional boolean centerIfNeeded);