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

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below. The name of the top level entry should be of the form <policy name>[.suffix]. The optional suffix is used for defining multiple test cases for a single policy.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "android", "Defaults to empty if not specified."],
     "official_only": "Whether this policy exists in official builds only. Defaults to |false| if not specified.",
     "can_be_recommended": "Whether a recommended value may be set for the policy. Defaults to |false| if not specified.",
     "test_policy": "A policy dictionary that should make the preferences affected by this policy become policy-controlled. Usually just sets the current policy. Defaults to an empty dictionary if not specified.",
     "note": "If the policy affects any preferences, the following array should be specified with one entry per such preference.",
     "pref_mappings": [
@@ skipping 3005 matching lines @@
   },
 
   "LoginApps": {
     "os": ["chromeos"],
     "test_policy": { "LoginApps": [ "abcdefghijklmnopabcdefghijklmnop" ] },
     "pref_mappings": [
       { "pref": "cros.device.login_apps" }
     ]
   },
 
+  "DeviceWallpaperImage": {
+    "os": ["chromeos"],
+    "test_policy": {
+      "DeviceWallpaperImage": {
+        "url": "http://localhost/",
+        "hash": "examplewallpaperhash"
+      }
+    },
+    "pref_mappings": [
+      { "pref": "cros.device_wallpaper_image" }
+    ]
+  },
+
   "----- Chrome Frame policies -------------------------------------------": {},
 
   "ChromeFrameRendererSettings": {
   },
 
   "RenderInChromeFrameList": {
   },
 
   "RenderInHostList": {
   },
 
   "ChromeFrameContentTypes": {
   },
 
   "GCFUserDataDir": {
   },
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }