IDL: Clean-up "should use Nullable<>" logic a little

Source/bindings/scripts/v8_types.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 378 matching lines @@
 
 def includes_for_interface(interface_name):
     return IdlType(interface_name).includes_for_type
 
 
 def add_includes_for_interface(interface_name):
     includes.update(includes_for_interface(interface_name))
 
 
 def impl_should_use_nullable_container(idl_type):
-    return idl_type.native_array_element_type or idl_type.is_primitive_type
+    return not(idl_type.cpp_type_has_null_value)
 
 IdlTypeBase.impl_should_use_nullable_container = property(
     impl_should_use_nullable_container)
 
 
 def impl_includes_for_type(idl_type, interfaces_info):
     includes_for_type = set()
     if idl_type.impl_should_use_nullable_container:
         includes_for_type.add('bindings/core/v8/Nullable.h')
 
@@ skipping 373 matching lines @@
     return literal_value
 
 IdlType.literal_cpp_value = literal_cpp_value
 
 
 ################################################################################
 # Utility properties for nullable types
 ################################################################################
 
 
+def cpp_type_has_null_value(idl_type):
+    # - String types (String/AtomicString) represent null as a null string,
+    #   i.e. one for which String::isNull() returns true.
+    # - Wrapper types (raw pointer or RefPtr/PassRefPtr) represent null as
+    #   a null pointer.
+    return ((idl_type.is_string_type or idl_type.is_wrapper_type) and
+            not idl_type.native_array_element_type)
+
+IdlTypeBase.cpp_type_has_null_value = property(cpp_type_has_null_value)
+
+
 def is_implicit_nullable(idl_type):
     # Nullable type where the corresponding C++ type supports a null value.
-    # - String types (String/AtomicString) represent null as a null string,
-    #   i.e. one for which String::isNull() returns true.
-    # - Wrapper types (raw pointer or RefPtr/PassRefPtr) represent null as
-    #   a null pointer.
-    return idl_type.is_nullable and (
-        (idl_type.is_string_type or idl_type.is_wrapper_type) and
-        not idl_type.native_array_element_type)
+    return idl_type.is_nullable and idl_type.cpp_type_has_null_value
 
 
 def is_explicit_nullable(idl_type):
     # Nullable type that isn't implicit nullable (see above.) For such types,
     # we use Nullable<T> or similar explicit ways to represent a null value.
     return idl_type.is_nullable and not idl_type.is_implicit_nullable
 
 IdlTypeBase.is_implicit_nullable = property(is_implicit_nullable)
 IdlUnionType.is_implicit_nullable = False
 IdlTypeBase.is_explicit_nullable = property(is_explicit_nullable)