Add a SigninAllowed policy.

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "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 342 matching lines @@
   },
 
   "SyncDisabled": {
     "os": ["win", "linux", "mac", "chromeos"],
     "test_policy": { "SyncDisabled": true },
     "pref_mappings": [
       { "pref": "sync.managed" }
     ]
   },
 
+  "SigninAllowed": {
+    "os": ["win", "linux", "mac"],
+    "test_policy": { "SigninAllowed": true },
+    "pref_mappings": [
+      { "pref": "signin.allowed" }
+    ]
+  },
+  
   "UserDataDir": {
     "note": "TODO(joaodasilva): To test that this policy works correctly, it would need to be set before the browser is launched. PolicyPrefsTest should be refactored to support this and a test for this policy added."
   },
 
   "DiskCacheDir": {
     "os": ["win", "mac", "linux"],
     "test_policy": { "DiskCacheDir": "${user_home}/test-cache" },
     "pref_mappings": [
       { "pref": "browser.disk_cache_dir" }
     ]
@@ skipping 1439 matching lines @@
 
   "ChromeFrameContentTypes": {
   },
 
   "GCFUserDataDir": {
   },
 
   "AdditionalLaunchParameters": {
   }
 }