WIP bindings: Add support for the record<K,V> WebIDL type.

third_party/WebKit/Source/bindings/templates/callback_function.h.tmpl

 {% filter format_blink_cpp_source_code %}
 
 {% include 'copyright_block.txt' %}
 
 #ifndef {{cpp_class}}_h
 #define {{cpp_class}}_h
 
 {% for filename in header_includes %}
 #include "{{filename}}"
 {% endfor %}
@@ skipping 20 matching lines @@
     return m_callback.newLocal(isolate);
   }
 
  private:
   {{cpp_class}}(ScriptState*, v8::Local<v8::Function>);
 
   RefPtr<ScriptState> m_scriptState;
   TraceWrapperV8Reference<v8::Function> m_callback;
 };
 
+template <>
+struct NativeValueTraits<{{cpp_class}}> {
+  using ImplType = {{cpp_class}};
+  {{exported}}static {{cpp_class}}* nativeValue(v8::Isolate*, v8::Local<v8::Value>, ExceptionState&);
+};
+
 }  // namespace blink
 
 #endif  // {{cpp_class}}_h
 
 {% endfilter %}{# format_blink_cpp_source_code #}