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

components/signin/core/browser/signin_manager.cc

 // 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.
 
 #include "components/signin/core/browser/signin_manager.h"
 
 #include <string>
 #include <vector>
 
 #include "base/metrics/histogram.h"
@@ skipping 338 matching lines @@
 }
 
 const std::string& SigninManager::GetUsernameForAuthInProgress() const {
   return possibly_invalid_username_;
 }
 
 void SigninManager::DisableOneClickSignIn(PrefService* prefs) {
   prefs->SetBoolean(prefs::kReverseAutologinEnabled, false);
 }
 
+void SigninManager::MergeSigninCredentialIntoCookieJar() {
+  if (!client_->ShouldMergeSigninCredentialsIntoCookieJar())
+    return;
+
+  if (!IsAuthenticated())
+    return;
+
+  // Don't execute 2 MergeSessionHelpers. New account takes priority.
+  if (merge_session_helper_.get() && merge_session_helper_->is_running())
+    merge_session_helper_->CancelAll();
+
+  merge_session_helper_.reset(new MergeSessionHelper(
+      token_service_, GaiaConstants::kChromeSource,
+      client_->GetURLRequestContext(), NULL));
+
+  merge_session_helper_->LogIn(GetAuthenticatedAccountId());
+}
+
 void SigninManager::CompletePendingSignin() {
   DCHECK(!possibly_invalid_username_.empty());
   OnSignedIn(possibly_invalid_username_);
 
-  if (client_->ShouldMergeSigninCredentialsIntoCookieJar()) {
-    merge_session_helper_.reset(new MergeSessionHelper(
-        token_service_, GaiaConstants::kChromeSource,
-        client_->GetURLRequestContext(), NULL));
-  }
-
   DCHECK(!temp_refresh_token_.empty());
   DCHECK(IsAuthenticated());
-  std::string account_id = GetAuthenticatedAccountId();
-  token_service_->UpdateCredentials(account_id, temp_refresh_token_);
+  token_service_->UpdateCredentials(GetAuthenticatedAccountId(),
+                                    temp_refresh_token_);
   temp_refresh_token_.clear();
 
-  if (client_->ShouldMergeSigninCredentialsIntoCookieJar())
-    merge_session_helper_->LogIn(account_id);
+  MergeSigninCredentialIntoCookieJar();
 }
 
 void SigninManager::OnExternalSigninCompleted(const std::string& username) {
   OnSignedIn(username);
 }
 
 void SigninManager::OnSignedIn(const std::string& username) {
   client_->GetPrefs()->SetInt64(prefs::kSignedInTime,
                                 base::Time::Now().ToInternalValue());
   SetAuthenticatedUsername(username);
@@ skipping 38 matching lines @@
 void SigninManager::OnAccountUpdateFailed(const std::string& account_id) {
   user_info_fetched_by_account_tracker_ = true;
   PostSignedIn();
 }
 
 void SigninManager::ProhibitSignout(bool prohibit_signout) {
   prohibit_signout_ = prohibit_signout;
 }
 
 bool SigninManager::IsSignoutProhibited() const { return prohibit_signout_; }