IDL: Support optional argument default value syntax

Source/bindings/v8/V8Binding.cpp

Index: Source/bindings/v8/V8Binding.cpp
diff --git a/Source/bindings/v8/V8Binding.cpp b/Source/bindings/v8/V8Binding.cpp
index bef984e7af5aff1ac0aafda305d59436dbe6173d..88192b78c1754962653549e4a784cd6ae0fe0b84 100644
--- a/Source/bindings/v8/V8Binding.cpp
+++ b/Source/bindings/v8/V8Binding.cpp
@@ -494,7 +494,7 @@ float toFloat(v8::Handle<v8::Value> value, ExceptionState& exceptionState)
 
 String toByteString(v8::Handle<v8::Value> value, ExceptionState& exceptionState)
 {
-    // Handle [Default=NullString]
+    // Handle null default value.
     if (value.IsEmpty())
         return String();