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

components/policy/resources/policy_templates.json

 {
 # policy_templates.json - Metafile for policy templates
 #
 # The content of this file is evaluated as a Python expression.
 #
 # This file is used as input to generate the following policy templates:
 # ADM, ADMX+ADML, MCX/plist and html documentation.
 #
 # Policy templates are user interface definitions or documents about the
 # policies that can be used to configure Chrome. Each policy is a name-value
@@ skipping 121 matching lines @@
 #   templates and documentation. The policy definition list that Chrome sees
 #   will include policies marked with 'future'. If a WIP policy isn't meant to
 #   be seen by the policy providers either, the 'supported_on' key should be set
 #   to an empty list.
 #
 # IDs:
 #   Since a Protocol Buffer definition is generated from this file, unique and
 #   persistent IDs for all fields (but not for groups!) are needed. These are
 #   specified by the 'id' keys of each policy. NEVER CHANGE EXISTING IDs,
 #   because doing so would break the deployed wire format!
-#   For your editing convenience: highest ID currently used: 356
+#   For your editing convenience: highest ID currently used: 357
 #
 # Placeholders:
 #   The following placeholder strings are automatically substituted:
 #     $1 -> Google Chrome / Chromium
 #     $2 -> Google Chrome OS / Chromium OS
 #     $3 -> Google Chrome Frame / Chromium Frame
 #     $6 is reserved for doc_writer
 #
 # Device Policy:
 #   An additional flag 'device_only' (optional, defaults to False) indicates
@@ skipping 9179 matching lines @@
           'example_value': False,
           'id': 356,
           'caption': '''Enables users to set weak PINs for the lock screen PIN.''',
           'tags': [],
           'desc': '''If this setting is configured to be false, users will be prevented from setting PINs that are considered weak. Such PINs include PINs which contain the same digit throughout, PINs whose digits are increasing by 1, PINs whose digits are decreasing by 1 and PINs which are too common.
 
           If this setting is configured to be true, or not configured, users can enter these PINs and will only get a warning display.''',
         },
       ],
     },
+    {
+      'name': 'DeviceWallpaperImage',
+      'type': 'external',
+      'schema': {
+        'type': 'object',
+        'properties': {
+          'url': { "type": "string" },
+          'hash': { "type": "string" }
+        },
+      },
+      'supported_on': ['chrome_os:57-'],
+      'device_only': True,
+      'features': {
+        'dynamic_refresh': True,
+      },
+      'example_value': { "url": "https://example.com/device_wallpaper.jpg", "hash": "examplewallpaperexamplewallpaperexamplewallpaperexamplewallpaper" },
+      'max_size': 16777216,
+      'id': 357,
+      'caption': '''Device wallpaper image''',
+      'tags': [],
+      'desc': '''Configure device-level wallpaper image that is shown on the login screen if no user has yet signed in to the device. The policy is set by specifying the URL from which the Chrome OS device can download the wallpaper image and a cryptographic hash used to verify the integrity of the download. The image must be in JPEG format, its file size must not exceed 16MB. The URL must be accessible without any authentication. The wallpaper image is downloaded and cached. It will be re-downloaded whenever the URL or the hash changes.
+
+      The policy should be speicified as a string that expresses the URL and hash in JSON format, e.g.,
+      {
+        "url": "https://example.com/device_wallpaper.jpg",
+        "hash": "examplewallpaperhash"
+      }
+
+      If the device wallpaper policy is set, the Chrome OS device will download and use the wallpaper image on the login screen if no user has yet signed in to the device. Once the user logs in, the user's wallpaper policy kicks in.
+
+      If the device wallpaper policy is left not set, it's the user's wallpaper policy to decide what to show if the user's wallpaper policy is set.''',
+    },
   ],
   'messages': {
     # Messages that are not associated to any policies.
     'win_supported_winxpsp2': {
       'desc': '''A label specifying the oldest possible compatible version of Windows. This text will appear right next to a label containing the text 'Supported on:'.''',
       'text': '''Microsoft Windows XP SP2 or later'''
     },
     'mac_chrome_preferences': {
       'desc': '''A text indicating in Mac OS X Workgroup Manager, that currently the preferences of Chromium are being edited''',
       'text': '''<ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> preferences'''
@@ skipping 113 matching lines @@
       'desc': '''Text appended in parentheses next to the policies top-level container to indicate that those policies are of the Recommended level''',
       'text': 'Default Settings (users can override)',
     },
     'doc_complex_policies_on_windows': {
       'desc': '''Text pointing the user to a help article for complex policies on Windows''',
       'text': '''encoded as a JSON string, for details see <ph name="COMPLEX_POLICIES_URL">https://www.chromium.org/administrators/complex-policies-on-windows<ex>https://www.chromium.org/administrators/complex-policies-on-windows</ex></ph>''',
     },
   },
   'placeholders': [],
 }