Reauth causes a MergeSession, so the cookie jar gets re-authed too.

components/signin/core/browser/signin_manager.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // The signin manager encapsulates some functionality tracking
 // which user is signed in. See SigninManagerBase for full description of
 // responsibilities. The class defined in this file provides functionality
 // required by all platforms except Chrome OS.
 //
 // When a user is signed in, a ClientLogin request is run on their behalf.
@@ skipping 82 matching lines @@
   // Sign a user out, removing the preference, erasing all keys
   // associated with the user, and canceling all auth in progress.
   virtual void SignOut(signin_metrics::ProfileSignout signout_source_metric);
 
   // On platforms where SigninManager is responsible for dealing with
   // invalid username policy updates, we need to check this during
   // initialization and sign the user out.
   void Initialize(PrefService* local_state) override;
   void Shutdown() override;
 
+  // If applicable, merge the signed in account into the cookie jar.
+  void MergeSigninCredentialIntoCookieJar();
+
   // Invoked from an OAuthTokenFetchedCallback to complete user signin.
   virtual void CompletePendingSignin();
 
   // Invoked from SigninManagerAndroid to indicate that the sign-in process
   // has completed for |username|.
   void OnExternalSigninCompleted(const std::string& username);
 
   // Returns true if there's a signin in progress.
   bool AuthInProgress() const override;
 
@@ skipping 120 matching lines @@
   bool user_info_fetched_by_account_tracker_;
 
   base::WeakPtrFactory<SigninManager> weak_pointer_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SigninManager);
 };
 
 #endif  // !defined(OS_CHROMEOS)
 
 #endif  // COMPONENTS_SIGNIN_CORE_BROWSER_SIGNIN_MANAGER_H_