Mojo C++ bindings: add support for UnionTraits.

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.
 
 static_library("bindings") {
   sources = [
     "array.h",
     "array_data_view.h",
     "array_traits.h",
     "array_traits_carray.h",
@@ skipping 106 matching lines @@
     "string_traits_standard.h",
     "string_traits_stl.h",
     "string_traits_string16.h",
     "string_traits_string_piece.h",
     "strong_binding.h",
     "struct_ptr.h",
     "sync_call_restrictions.h",
     "sync_handle_registry.h",
     "sync_handle_watcher.h",
     "type_converter.h",
+    "union_traits.h",
   ]
 
   public_deps = [
     ":struct_traits",
     "//base",
     "//ipc:param_traits",
     "//mojo/public/cpp/system",
   ]
 
   deps = [
@@ skipping 25 matching lines @@
     ]
 
     public_deps = [
       ":bindings",
       "//third_party/WebKit/Source/wtf",
     ]
 
     public_configs = [ "//third_party/WebKit/Source:config" ]
   }
 }