IDL: Support optional argument default value syntax

Source/bindings/tests/idls/TestObject.idl

Index: Source/bindings/tests/idls/TestObject.idl
diff --git a/Source/bindings/tests/idls/TestObject.idl b/Source/bindings/tests/idls/TestObject.idl
index 9023e06b5b80716aeee46003463cbcb77a6c65a1..3ee6893479797150338052b6532c8cdf84450203 100644
--- a/Source/bindings/tests/idls/TestObject.idl
+++ b/Source/bindings/tests/idls/TestObject.idl
@@ -376,6 +376,16 @@ interface TestObject {
     // Optional arguments: exceptional case
     void voidMethodOptionalDictionaryArg(optional Dictionary optionalDictionaryArg);
 
+    // Optional arguments with defaults
+    void voidMethodDefaultStringArg(optional DOMString defaultStringArg = "foo");
+    void voidMethodDefaultNullStringArg(optional DOMString defaultStringArg = null);
+    void voidMethodDefaultLongArg(optional long defaultLongArg = 10);
+    void voidMethodDefaultDoubleArg(optional double defaultDoubleArg = 0.5);
+    void voidMethodDefaultTrueBooleanArg(optional boolean defaultBooleanArg = true);
+    void voidMethodDefaultFalseBooleanArg(optional boolean defaultBooleanArg = false);
+    void voidMethodDefaultNullableStringArg(optional DOMString? defaultStringArg = null);
+    void voidMethodDefaultNullableTestInterfaceArg(optional TestInterface? defaultTestInterfaceArg = null);
+
     // Variadic operations
     void voidMethodVariadicStringArg(DOMString... variadicStringArgs);
     void voidMethodStringArgVariadicStringArg(DOMString stringArg, DOMString... variadicStringArgs);
@@ -398,7 +408,7 @@ interface TestObject {
     void overloadedMethodF(optional DOMString stringArg);
     void overloadedMethodF(double doubleArg);
     void overloadedMethodG(long longArg);
-    void overloadedMethodG([Default=Null] TestInterfaceEmpty? testInterfaceEmptyOrNullArg);
+    void overloadedMethodG(optional TestInterfaceEmpty? testInterfaceEmptyOrNullArg = null);
     void overloadedMethodH(TestInterface testInterfaceArg);
     void overloadedMethodH(TestInterfaceEmpty testInterfaceEmptyArg);
     void overloadedMethodI(DOMString stringArg);
@@ -419,7 +429,6 @@ interface TestObject {
     void voidMethodDefaultUndefinedTestInterfaceEmptyArg([Default=Undefined] optional TestInterfaceEmpty defaultUndefinedTestInterfaceEmptyArg);
     void voidMethodDefaultUndefinedLongArg([Default=Undefined] optional long defaultUndefinedLongArg);
     void voidMethodDefaultUndefinedStringArg([Default=Undefined] optional DOMString defaultUndefinedStringArg);
-    void voidMethodDefaultNullStringStringArg([Default=NullString] optional DOMString defaultNullStringStringArg);
     // [EnforceRange]
     void voidMethodEnforceRangeLongArg([EnforceRange] long enforceRangeLongArg);
     // [TreatNullAs], [TreatUndefinedAs]