[ios] Fix "return foo = nil" introduced by ARC conversion tools.

ios/chrome/browser/ui/settings/sync_encryption_passphrase_collection_view_controller.mm

 // Copyright 2015 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.
 
 #import "ios/chrome/browser/ui/settings/sync_encryption_passphrase_collection_view_controller.h"
 
 #include <memory>
 
 #include "base/i18n/time_formatting.h"
 #include "base/mac/foundation_util.h"
@@ skipping 463 matching lines @@
   spinnerFrame.origin.y = kSpinnerButtonPadding;
   [spinner setFrame:spinnerFrame];
   [customView addSubview:spinner];
 
   UIBarButtonItem* leftBarButtonItem =
       [[UIBarButtonItem alloc] initWithCustomView:customView];
 
   [spinner setHidesWhenStopped:NO];
   [spinner startAnimating];
 
-  return leftBarButtonItem = nil;
+  return leftBarButtonItem;
 }
 
 - (void)stopObserving {
   // Stops observing the sync service. This is required during the shutdown
   // phase to avoid observing sync events for a browser state that is being
   // killed.
   syncObserver_.reset();
   tokenServiceObserver_.reset();
 }
 
@@ skipping 72 matching lines @@
       self.navigationController) popViewControllerOrCloseSettingsAnimated:NO];
 }
 
 #pragma mark - SettingsControllerProtocol callbacks
 
 - (void)settingsWillBeDismissed {
   [self stopObserving];
 }
 
 @end