[Chrome OS] Add a chrome os device wallpaper policy.

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

 // Copyright 2013 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
@@ skipping 736 matching lines @@
   optional bool enabled = 1 [default = false];
   optional int32 upload_rate_kbits = 2 [default = 0];
   optional int32 download_rate_kbits = 3 [default = 0];
 }
 
 // A list of app-ids to install from the webstore on the login page.
 message LoginAppsProto {
   repeated string login_apps = 1;
 }
 
+// The url and hash that specified in JSON format that can be used to set the
+// device-level wallpaper on the login screen before any user logs in.
+message DeviceWallpaperImageProto {
+  optional string device_wallpaper_image = 1;
+}
+
 message ChromeDeviceSettingsProto {
   optional DevicePolicyRefreshRateProto device_policy_refresh_rate = 1;
   optional UserWhitelistProto user_whitelist = 2;
   optional GuestModeEnabledProto guest_mode_enabled = 3;
   optional OBSOLETE_DeviceProxySettingsProto device_proxy_settings = 4
       [deprecated = true];
   optional CameraEnabledProto camera_enabled = 5;
   optional ShowUserNamesOnSigninProto show_user_names = 6;
   optional DataRoamingEnabledProto data_roaming_enabled = 7;
   optional AllowNewUsersProto allow_new_users = 8;
@@ skipping 34 matching lines @@
   optional AllowKioskAppControlChromeVersionProto
       allow_kiosk_app_control_chrome_version = 40;
   optional LoginAuthenticationBehaviorProto login_authentication_behavior = 41;
   optional UsbDetachableWhitelistProto usb_detachable_whitelist = 42;
   optional AllowBluetoothProto allow_bluetooth = 43;
   optional DeviceQuirksDownloadEnabledProto quirks_download_enabled = 44;
   optional LoginVideoCaptureAllowedUrlsProto login_video_capture_allowed_urls =
       45;
   optional LoginAppsProto login_apps = 46;
   optional NetworkThrottlingEnabledProto network_throttling = 47;
+  optional DeviceWallpaperImageProto device_wallpaper_image = 48;
 }