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

chromeos/settings/cros_settings_names.cc

Index: chromeos/settings/cros_settings_names.cc
diff --git a/chromeos/settings/cros_settings_names.cc b/chromeos/settings/cros_settings_names.cc
index 094a6c75254a8a76d29e85c165fd08d1918943b8..dec80cf6db04be0f8d03c4053f2619c0a43c5193 100644
--- a/chromeos/settings/cros_settings_names.cc
+++ b/chromeos/settings/cros_settings_names.cc
@@ -227,4 +227,8 @@ const char kLoginVideoCaptureAllowedUrls[] =
 // A list pref storing the apps to install on the login page.
 const char kLoginApps[] = "cros.device.login_apps";
 
+// A string pref storing the url and cryptographic hash of the image in JSON
+// format allowed to set a device-level wallpaper before any user logs in.
+const char kDeviceWallpaperImage[] = "cros.device_wallpaper_image";
+
 }  // namespace chromeos