Add a SigninAllowed policy.

chrome/browser/policy/configuration_policy_handler.h

Index: chrome/browser/policy/configuration_policy_handler.h
diff --git a/chrome/browser/policy/configuration_policy_handler.h b/chrome/browser/policy/configuration_policy_handler.h
index 8d75ddfb32c98bcae425a2e0c5b2cd1e15846c48..4a22cdf1efe901b415648d245915b5b5dc9f1bcd 100644
--- a/chrome/browser/policy/configuration_policy_handler.h
+++ b/chrome/browser/policy/configuration_policy_handler.h
@@ -221,6 +221,20 @@ class SyncPolicyHandler : public TypeCheckingPolicyHandler {
   DISALLOW_COPY_AND_ASSIGN(SyncPolicyHandler);
 };
 
+// ConfigurationPolicyHandler for the SigninAllowed policy.
+class SigninPolicyHandler: public TypeCheckingPolicyHandler {
+ public:
+  SigninPolicyHandler();
+  virtual ~SigninPolicyHandler();
+
+  // ConfigurationPolicyHandler methods:
+  virtual void ApplyPolicySettings(const PolicyMap& policies,
+                                   PrefValueMap* prefs) OVERRIDE;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(SigninPolicyHandler);
+};
+
 // ConfigurationPolicyHandler for the AutofillEnabled policy.
 class AutofillPolicyHandler : public TypeCheckingPolicyHandler {
  public: