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

chrome/test/data/policy/policy_test_cases.json

Index: chrome/test/data/policy/policy_test_cases.json
diff --git a/chrome/test/data/policy/policy_test_cases.json b/chrome/test/data/policy/policy_test_cases.json
index 63b3be9ad4b965bea202ac24c55ca75c0d98e919..dce735fb0e771cb0eb76c155b2363b07945760e3 100644
--- a/chrome/test/data/policy/policy_test_cases.json
+++ b/chrome/test/data/policy/policy_test_cases.json
@@ -3023,6 +3023,19 @@
     ]
   },
 
+  "DeviceWallpaperImage": {
+    "os": ["chromeos"],
+    "test_policy": {
+      "DeviceWallpaperImage": {
+        "url": "http://localhost/",
+        "hash": "examplewallpaperexamplewallpaperexamplewallpaperexamplewallpaper"

[pastarmovj, 2016/12/14 10:55:02]

nit: does this have to be so long to be accepted by the code or cane we go with
a single examplewallpaper isntead? :)

[xdai1, 2016/12/14 18:22:25]

It doesn't have to be so long to pass the test, so I reduced it to
"examplewallpaerhash" as you suggested. 

+      }
+    },
+    "pref_mappings": [
+      { "pref": "cros.device_wallpaper_image" }
+    ]
+  },
+
   "----- Chrome Frame policies -------------------------------------------": {},
 
   "ChromeFrameRendererSettings": {