Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/core/frame/Window.idl

Index: Source/core/frame/Window.idl
diff --git a/Source/core/frame/Window.idl b/Source/core/frame/Window.idl
index a742c817360dd24bbf231d80ab79077d8e194cb2..b644a8db8edbafece5dde78020029ba237e8dec4 100644
--- a/Source/core/frame/Window.idl
+++ b/Source/core/frame/Window.idl
@@ -140,11 +140,11 @@
 
     // DOM Level 2 Style Interface
     CSSStyleDeclaration getComputedStyle([Default=Undefined] optional Element element,
-                                                            [TreatNullAs=NullString, TreatUndefinedAs=NullString, Default=Undefined] optional DOMString pseudoElement);
+                                         [TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? pseudoElement);
 
     // WebKit extensions
     [MeasureAs=GetMatchedCSSRules] CSSRuleList getMatchedCSSRules([Default=Undefined] optional Element element,
-                                                                  [TreatNullAs=NullString, TreatUndefinedAs=NullString, Default=Undefined] optional DOMString pseudoElement);
+                                                                  [TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? pseudoElement);
 
     [Replaceable] readonly attribute double devicePixelRatio;