Mojo C++ bindings: support setting default value for fields whose types are typemapped enums.

mojo/public/cpp/bindings/tests/struct_traits_unittest.cc

 // Copyright 2015 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.
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
@@ skipping 532 matching lines @@
                             ->get_struct()
                             .value);
               quit_closure.Run();
 
             },
             loop.QuitClosure()));
     loop.Run();
   }
 }
 
+TEST_F(StructTraitsTest, DefaultValueOfEnumWithTraits) {
+  auto container = EnumWithTraitsContainer::New();
+  EXPECT_EQ(EnumWithTraitsImpl::CUSTOM_VALUE_1, container->f_field);
+}
+
 }  // namespace test
 }  // namespace mojo