Make keySystem argument optional in MediaElement.canPlayType.

tools/dom/idl/dart/dart.idl

Index: tools/dom/idl/dart/dart.idl
diff --git a/tools/dom/idl/dart/dart.idl b/tools/dom/idl/dart/dart.idl
index 907cbb0c4c336e8313587166c4c6b8102d4a39ea..289bcf60c3676762b4941b41c4a60d9e13866eba 100644
--- a/tools/dom/idl/dart/dart.idl
+++ b/tools/dom/idl/dart/dart.idl
@@ -184,6 +184,11 @@ interface IDBIndex {
 };
 
 [Supplemental]
+interface HTMLMediaElement {
+  DOMString canPlayType([Default=Undefined] optional DOMString type, [Default=Undefined, TreatNullAs=NullString, TreatUndefinedAs=NullString, ForceOptional] optional DOMString keySystem);
+};
+
+[Supplemental]
 interface IDBKeyRange {
   [DartName=only_] static IDBKeyRange only(any value);
   [DartName=lowerBound_] static IDBKeyRange lowerBound(any bound, [ForceOptional] optional boolean open);
@@ -319,4 +324,4 @@ interface Location {
   [CustomSetter] attribute DOMString pathname;
   [CustomSetter] attribute DOMString search;
   [CustomSetter] attribute DOMString hash;
-};
+};