[aura-mus] Add ui::CursorData, with mojo serialization.

services/ui/public/interfaces/cursor/BUILD.gn

+# 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.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+mojom("cursor") {
+  sources = [
+    "cursor.mojom",
+  ]
+
+  public_deps = [
+    "//mojo/common:common_custom_types",
+    "//skia/public/interfaces",
+    "//skia/public/interfaces:mojo",
+    "//ui/gfx/geometry/mojo",
+  ]
+}
+
+mojom("test_interfaces") {
+  sources = [
+    "cursor_struct_traits_test.mojom",
+  ]
+
+  public_deps = [
+    ":cursor",
+  ]
+}