Add field-initializing factory static methods to generated mojo unions.

mojo/public/cpp/bindings/BUILD.gn

 # 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.
 
 interfaces_bindings_gen_dir = "$root_gen_dir/mojo/public/interfaces/bindings"
 
 component("bindings") {
   sources = [
     # Normally, targets should depend on the source_sets generated by mojom
     # targets. However, the generated source_sets use portions of the bindings
@@ skipping 87 matching lines @@
     "lib/serialization_context.cc",
     "lib/serialization_context.h",
     "lib/serialization_forward.h",
     "lib/serialization_util.h",
     "lib/string_serialization.h",
     "lib/string_traits_string16.cc",
     "lib/sync_call_restrictions.cc",
     "lib/sync_handle_registry.cc",
     "lib/sync_handle_watcher.cc",
     "lib/template_util.h",
-    "lib/union_accessor.h",
     "lib/validate_params.h",
     "lib/validation_context.cc",
     "lib/validation_context.h",
     "lib/validation_errors.cc",
     "lib/validation_errors.h",
     "lib/validation_util.cc",
     "lib/validation_util.h",
     "map.h",
     "map_data_view.h",
     "map_traits.h",
@@ skipping 63 matching lines @@
     ]
 
     public_deps = [
       ":bindings",
       "//third_party/WebKit/Source/wtf",
     ]
 
     public_configs = [ "//third_party/WebKit/Source:config" ]
   }
 }