IDL: Generate trace() method in union container types when required

Source/bindings/templates/union.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 {{macro_guard}}
 #define {{macro_guard}}
 
 {% for filename in header_includes %}
@@ skipping 12 matching lines @@
 public:
     {{container.cpp_class}}();
     bool isNull() const { return m_type == SpecificTypeNone; }
 
     {% for member in container.members %}
     bool is{{member.type_name}}() const { return m_type == {{member.specific_type_enum}}; }
     {{member.rvalue_cpp_type}} getAs{{member.type_name}}();
     void set{{member.type_name}}({{member.rvalue_cpp_type}});
 
     {% endfor %}
+    {% if container.needs_trace %}
+    void trace(Visitor*);
+
+    {% endif %}
 private:
     enum SpecificTypes {
         SpecificTypeNone,
         {% for member in container.members %}
         {{member.specific_type_enum}},
         {% endfor %}
     };
     SpecificTypes m_type;
 
     {% for member in container.members %}
@@ skipping 11 matching lines @@
 template <class CallbackInfo>
 inline void v8SetReturnValue(const CallbackInfo& callbackInfo, {{container.cpp_class}}& impl)
 {
     v8SetReturnValue(callbackInfo, toV8(impl, callbackInfo.Holder(), callbackInfo.GetIsolate()));
 }
 
 {% endfor %}
 } // namespace blink
 
 #endif // {{macro_guard}}