reduce setting wallpaper latency.

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: 2
 
 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);
+  // |data| is a byte array of the wallpaper image.
+  SetWallpaper@1(array<uint8> data);
 };
 
 // Connects with container side to publish wallpaper related intents.
 // Next method ID:2
 interface WallpaperInstance {
   // Establishes communication with the container side.
   Init@0(WallpaperHost host_ptr);
 
   // Notifies ArcWallpaperManagerService that wallpaper is changed.
   [MinVersion=1] OnWallpaperChanged@1();
 };