IDL: Add [LegacyInterfaceTypeChecking] extended attribute

Source/bindings/tests/idls/core/TestInterface.idl

Index: Source/bindings/tests/idls/core/TestInterface.idl
diff --git a/Source/bindings/tests/idls/core/TestInterface.idl b/Source/bindings/tests/idls/core/TestInterface.idl
index 4c945efa43f1a9aa2a6c9d6e7c321801079f562b..ef800cc0f5f315d839d6bae562c035a49e3dd707 100644
--- a/Source/bindings/tests/idls/core/TestInterface.idl
+++ b/Source/bindings/tests/idls/core/TestInterface.idl
@@ -58,6 +58,7 @@
     attribute TestEnum testEnumAttribute;
     attribute DOMStringOrDouble stringOrDoubleAttribute;
     static attribute DOMString staticStringAttribute;
+    [LegacyInterfaceTypeChecking] attribute TestInterfaceEmpty legacyInterfaceTypeCheckingAttribute;
 
     void voidMethodTestInterfaceEmptyArg(TestInterfaceEmpty testInterfaceEmptyArg);
     void voidMethodDoubleArgFloatArg(double doubleArg, float floatArg);
@@ -106,6 +107,8 @@
     Promise promiseMethodPartialOverload();
     Promise promiseMethodPartialOverload(Window window);
     static Promise staticPromiseMethodPartialOverload();
+
+    [LegacyInterfaceTypeChecking] void legacyInterfaceTypeCheckingMethod(TestInterfaceEmpty testInterfaceEmptyArg);
 };
 
 TestInterface implements TestImplements;