IDL: Put generated union type containers in separate files

Source/bindings/templates/templates.gypi

 # 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.
 
 {
   'variables': {
     'code_generator_template_files': [
       'attributes.cpp',
       'callback_interface.cpp',
       'callback_interface.h',
       'constants.cpp',
       'dictionary_impl.cpp',
       'dictionary_impl.h',
       'dictionary_v8.cpp',
       'dictionary_v8.h',
       'interface_base.cpp',
       'interface.cpp',
       'interface.h',
       'methods.cpp',
       'partial_interface.cpp',
       'partial_interface.h',
       'union.cpp',
       'union.h',
+      'union_container.cpp',
+      'union_container.h',
     ],
   },
 }