cheets: implement cros side of WallpaperManagerService.

components/arc/common/wallpaper.mojom

+// Copyright 2016 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.
+//
+// Next MinVersion: 1
+
+module arc.mojom;
+
+// Handles wallpaper calls from ARC in Chrome.
+// Next method ID:2
+interface WallpaperHost {
+  // Gets current wallpaper encoded in PNG and send it back to ARC.
+  GetWallpaper@0() => (array<uint8> wallpaper);
+
+  // Sets an image from ARC as the wallpaper.
+  // |png_data| is a PNG encoded wallpaper image.
+  SetWallpaper@1(array<uint8> png_data);
+};
+
+// Connects with container side to publish wallpaper related intents.
+// Next method ID:1
+interface WallpaperInstance {
+  // Establishes communication with the container side.
+  Init@0(WallpaperHost host_ptr);
+};