Document attribute charset and its aliases to return UTF-8

Source/core/dom/Document.idl

Index: Source/core/dom/Document.idl
diff --git a/Source/core/dom/Document.idl b/Source/core/dom/Document.idl
index 5ebbf1193b4b5b5263b07f1695ad3d83ce30db84..a22b8f5d81b9bd5b0b23f1e0fc17ab298fb88e37 100644
--- a/Source/core/dom/Document.idl
+++ b/Source/core/dom/Document.idl
@@ -197,8 +197,8 @@ interface Document : Node {
     readonly attribute VisibilityState visibilityState;
 
     // Non-standard APIs
-    [MeasureAs=DocumentCharset, TreatReturnedNullStringAs=Undefined] readonly attribute DOMString charset;
-    [MeasureAs=DocumentDefaultCharset, TreatReturnedNullStringAs=Undefined] readonly attribute DOMString defaultCharset;
+    [MeasureAs=DocumentCharset] readonly attribute DOMString charset;
+    [MeasureAs=DocumentDefaultCharset] readonly attribute DOMString defaultCharset;
     [MeasureAs=DocumentCaretRangeFromPoint] Range caretRangeFromPoint([Default=Undefined] optional long x, [Default=Undefined] optional long y);
     [DeprecateAs=DocumentGetCSSCanvasContext] RenderingContext getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);