Add DisplayPlacement/Layout mojoms + StructTraits.

ui/display/mojo/display_layout.mojom

+// 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;
+
+// Corresponds to display::DisplayPlacement::Position.
+enum Position {
+  TOP,
+  RIGHT,
+  BOTTOM,
+  LEFT,
+};
+
+// Corresponds to display::DisplayPlacement::OffsetReference.
+enum OffsetReference {
+  TOP_LEFT,
+  BOTTOM_RIGHT,
+};
+
+// Corresponds to display::DisplayPlacement.
+struct DisplayPlacement {
+  int64 display_id;
+  int64 parent_display_id;
+  Position position;
+  int32 offset;
+  OffsetReference offset_reference;
+};
+
+// Corresponds to display::DisplayLayout.
+struct DisplayLayout {
+  bool mirrored;
+  bool default_unified;
+  int64 primary_display_id;
+  array<DisplayPlacement> placement_list;
+};