Add a SigninAllowed 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 17478f9a71c8697d4d5cde993a54239b9eb19848..723254684ba1acadc48310e7236419ec690e272e 100644
--- a/chrome/test/data/policy/policy_test_cases.json
+++ b/chrome/test/data/policy/policy_test_cases.json
@@ -360,6 +360,14 @@
     ]
   },
 
+  "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."
   },