Add a SigninAllowed policy.

chrome/browser/profiles/profile_io_data.h

Index: chrome/browser/profiles/profile_io_data.h
diff --git a/chrome/browser/profiles/profile_io_data.h b/chrome/browser/profiles/profile_io_data.h
index 39c9c3b22f988d063a0c46e85b086d20ecdb31c2..4cc4568c3f83c09492cabb759e7c6565ddec3d80 100644
--- a/chrome/browser/profiles/profile_io_data.h
+++ b/chrome/browser/profiles/profile_io_data.h
@@ -175,6 +175,10 @@ class ProfileIOData {
     return &sync_disabled_;
   }
 
+  BooleanPrefMember* signin_allowed() const {
+    return &signin_allowed_;
+  }
+
   net::TransportSecurityState* transport_security_state() const {
     return transport_security_state_.get();
   }
@@ -489,6 +493,7 @@ class ProfileIOData {
   mutable BooleanPrefMember safe_browsing_enabled_;
   mutable BooleanPrefMember printing_enabled_;
   mutable BooleanPrefMember sync_disabled_;
+  mutable BooleanPrefMember signin_allowed_;
   // TODO(marja): Remove session_startup_pref_ if no longer needed.
   mutable IntegerPrefMember session_startup_pref_;