IDL: Basic dictionary inheritance support

Source/bindings/templates/dictionary_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file has been auto-generated by {{code_generator}}. DO NOT MODIFY!
 
 #ifndef {{cpp_class}}_h
 #define {{cpp_class}}_h
 
 {% for filename in header_includes %}
 #include "{{filename}}"
 {% endfor %}
 
 namespace blink {
 
-class {{cpp_class}} final {
+{# FIXME: Add "final" if this class doesn't have subclasses #}
+class {{cpp_class}}{% if parent_cpp_class %} : public {{parent_cpp_class}}{% endif %} {
     ALLOW_ONLY_INLINE_ALLOCATION();
 public:
     {{cpp_class}}();
 
     {% for member in members %}
     bool {{member.has_method_name}}() const { return {{member.has_method_expression}}; }
     {{member.rvalue_cpp_type}} {{member.cpp_name}}() const { return {{member.getter_expression}}; }
     void {{member.setter_name}}({{member.rvalue_cpp_type}} value) { m_{{member.cpp_name}} = value; }
 
     {% endfor %}
-    void trace(Visitor*);
+    virtual void trace(Visitor*);
 
 private:
     {% for member in members %}
     {{member.member_cpp_type}} m_{{member.cpp_name}};
     {% endfor %}
 
     friend class V8{{cpp_class}};
 };
 
 } // namespace blink
 
 #endif // {{cpp_class}}_h