Add a SigninAllowed policy.

chrome/app/policy/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 94 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: 171
+#   For your editing convenience: highest ID currently used: 172
 #
 # 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 712 matching lines @@
       'example_value': True,
       'id': 20,
       'caption': '''Disable synchronization of data with Google''',
       'desc': '''Disables data synchronization in <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> using Google-hosted synchronization services and prevents users from changing this setting.
 
       If you enable this setting, users cannot change or override this setting in <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>.
 
       If this policy is left not set Google Sync will be available for the user to choose whether to use it or not.''',
     },
     {
+      'name': 'SigninAllowed',
+      'type': 'main',
+      'schema': { 'type': 'boolean' },
+      'supported_on': ['chrome.*:26-'],
+      'features': {
+        'dynamic_refresh': True,
+        'per_profile': True,
+      },
+      'example_value': True,
+      'id': 172,
+      'caption': '''Allows sign in to Chrome''',
+      'desc': '''Allows the user to sign in to <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> and prevents users from changing this setting.
+
+      If you set this policy, you can configure if a user is allowed to sign in to <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> or not.''',
+    },
+    {
       'name': 'UserDataDir',
       'type': 'string',
       'schema': { 'type': 'string' },
       'supported_on': ['chrome.win:11-', 'chrome.mac:11-'],
       'features': {
         'dynamic_refresh': False,
         'per_profile': False,
       },
       'example_value': '${users}/${user_name}/Chrome',
       'id': 63,
@@ skipping 3111 matching lines @@
       'desc': '''Text appended in parentheses to the policy name to indicate that it has been deprecated''',
       'text': 'deprecated',
     },
     'doc_recommended': {
       '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)',
     },
   },
   'placeholders': [],
 }