IDL: Support default values for dictionary members of union types

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}}"
 
 {% macro assign_and_return_if_hasinstance(member) %}
@@ skipping 29 matching lines @@
     String string = value;
     if (!({{member.enum_validation_expression}})) {
         ASSERT_NOT_REACHED();
         return;
     }
     {% endif %}
     m_{{member.cpp_name}} = value;
     m_type = {{member.specific_type_enum}};
 }
 
+{{container.cpp_class}} {{container.cpp_class}}::from{{member.type_name}}({{member.rvalue_cpp_type}} value)
+{
+    {{container.cpp_class}} container;
+    container.set{{member.type_name}}(value);
+    return container;
+}
+
 {% 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 %}
@@ skipping 121 matching lines @@
 
 {{container.cpp_class}} NativeValueTraits<{{container.cpp_class}}>::nativeValue(const v8::Local<v8::Value>& value, v8::Isolate* isolate, ExceptionState& exceptionState)
 {
     {{container.cpp_class}} impl;
     V8{{container.cpp_class}}::toImpl(isolate, value, impl, exceptionState);
     return impl;
 }
 
 {% endfor %}
 } // namespace blink