IDL: Treat undefined as missing for optional arguments with defaults

LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html

Index: LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html
diff --git a/LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html b/LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html
index 2bc8c504512f2f0d11bf0bf13b18a8ee429efc2e..12d149860bc66401b02c2e1dd2e5b1ee2973698f 100644
--- a/LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html
+++ b/LayoutTests/media/encrypted-media/encrypted-media-istypesupported.html
@@ -21,6 +21,14 @@
                         assert_true(type.length == 1 || type.length == 2);
                         assert_equals(MediaKeys.isTypeSupported.apply(
                             this, type), expectation, 'isTypeSupported');
+                        if (type.length == 1) {
+                            // Calling with undefined as the second argument
+                            // should behave same as calling with only one
+                            // argument.
+                            type.push(undefined);
+                            assert_equals(MediaKeys.isTypeSupported.apply(
+                                this, type), expectation, 'isTypeSupported');
+                        }
                     },  description + ": '" + type.join("', '") + "'",
                     { timeout: 60000 });
                 }