Add a SigninAllowed policy.

chrome/app/policy/policy_templates.json

Index: chrome/app/policy/policy_templates.json
diff --git a/chrome/app/policy/policy_templates.json b/chrome/app/policy/policy_templates.json
index 4c1fa136ffef71ead999715d8d8bd58970887d00..3a936d1ad65cec951f0e3a2597308596c16a3518 100644
--- a/chrome/app/policy/policy_templates.json
+++ b/chrome/app/policy/policy_templates.json
@@ -112,7 +112,7 @@
 #   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:
@@ -845,6 +845,22 @@
       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' },