IDL: Support default values for dictionary members of union types

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 10 matching lines @@
 class {{container.cpp_class}} final {
     ALLOW_ONLY_INLINE_ALLOCATION();
 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}}() const;
     void set{{member.type_name}}({{member.rvalue_cpp_type}});
+    static {{container.cpp_class}} from{{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 %}
@@ skipping 38 matching lines @@
         if (isUndefinedOrNull(v8Value))
             return;
         V8{{cpp_type}}::toImpl(isolate, v8Value, impl, exceptionState);
     }
 };
 
 {% endfor %}
 } // namespace blink
 
 #endif // {{macro_guard}}