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

Source/bindings/templates/union.cpp

 // 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!
 
 #include "config.h"
 #include "{{header_filename}}"
 
 {% for filename in cpp_includes %}
@@ skipping 16 matching lines @@
 }
 
 void {{container.cpp_class}}::set{{member.type_name}}({{member.rvalue_cpp_type}} value)
 {
     ASSERT(isNull());
     m_{{member.cpp_name}} = value;
     m_type = {{member.specific_type_enum}};
 }
 
 {% endfor %}
+{% if container.needs_trace %}
+void {{container.cpp_class}}::trace(Visitor* visitor)
+{
+    {% for member in container.members if member.is_traceable %}
+    visitor->trace(m_{{member.cpp_name}});
+    {% endfor %}
+}
+
+{% endif %}
 void V8{{container.cpp_class}}::toImpl(v8::Isolate* isolate, v8::Handle<v8::Value> v8Value, {{container.cpp_class}}& impl, ExceptionState& exceptionState)
 {
     {# FIXME: We don't follow the spec on handling null and undefined at this
        moment. Should be fixed once we implement all necessary conversion steps
        below. #}
     if (v8Value.IsEmpty() || isUndefinedOrNull(v8Value))
         return;
 
     {# The numbers in the following comments refer to the steps described in
        http://heycam.github.io/webidl/#es-union
@@ skipping 76 matching lines @@
     if (impl.is{{member.type_name}}())
         return {{member.cpp_value_to_v8_value}};
 
     {% endfor %}
     ASSERT_NOT_REACHED();
     return v8::Handle<v8::Value>();
 }
 
 {% endfor %}
 } // namespace blink