Write mojom and StructTraits for DisplaySnapshot.

ui/display/mojo/display_constants.mojom

Index: ui/display/mojo/display_constants.mojom
diff --git a/ui/display/mojo/display_constants.mojom b/ui/display/mojo/display_constants.mojom
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+// 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.
+
+module display.mojom;
+
+// Corresponding to display::DisplayConnectionType.
+enum DisplayConnectionType {
+  DISPLAY_CONNECTION_TYPE_NONE = 0,
+  DISPLAY_CONNECTION_TYPE_UNKNOWN = 1,
+  DISPLAY_CONNECTION_TYPE_INTERNAL = 2,
+  DISPLAY_CONNECTION_TYPE_VGA = 4,
+  DISPLAY_CONNECTION_TYPE_HDMI = 8,
+  DISPLAY_CONNECTION_TYPE_DVI = 16,
+  DISPLAY_CONNECTION_TYPE_DISPLAYPORT = 32,
+  DISPLAY_CONNECTION_TYPE_NETWORK = 64,
+  DISPLAY_CONNECTION_TYPE_VIRTUAL = 128,
+};