Write mojom and StructTraits for DisplaySnapshot.

ui/display/mojo/display_constants_struct_traits.h

Index: ui/display/mojo/display_constants_struct_traits.h
diff --git a/ui/display/mojo/display_constants_struct_traits.h b/ui/display/mojo/display_constants_struct_traits.h
new file mode 100644
index 0000000000000000000000000000000000000000..bb7c13eee31adfd0099c5e02553743ef942a8a95
--- /dev/null
+++ b/ui/display/mojo/display_constants_struct_traits.h
@@ -0,0 +1,24 @@
+// Copyright 2017 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.
+
+#ifndef UI_DISPLAY_MOJO_DISPLAY_CONSTANTS_STRUCT_TRAITS_H_
+#define UI_DISPLAY_MOJO_DISPLAY_CONSTANTS_STRUCT_TRAITS_H_
+
+#include "ui/display/mojo/display_constants.mojom.h"
+#include "ui/display/types/display_constants.h"
+
+namespace mojo {
+
+template <>
+struct EnumTraits<display::mojom::DisplayConnectionType,
+                  display::DisplayConnectionType> {
+  static display::mojom::DisplayConnectionType ToMojom(
+      display::DisplayConnectionType type);
+  static bool FromMojom(display::mojom::DisplayConnectionType type,
+                        display::DisplayConnectionType* out);
+};
+
+};  // namespace mojo
+
+#endif  // UI_DISPLAY_MOJO_DISPLAY_CONSTANTS_STRUCT_TRAITS_H_