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

ios/chrome/browser/ui/authentication/signin_account_selector_view_controller.mm

 // Copyright 2016 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 "ios/chrome/browser/ui/authentication/signin_account_selector_view_controller.h"
 
 #include <memory>
 
 #import "base/mac/foundation_util.h"
 #import "ios/chrome/browser/signin/chrome_identity_service_observer_bridge.h"
@@ skipping 113 matching lines @@
     @"divider" : divider,
     @"v1" : layoutGuide1,
     @"v2" : layoutGuide2
   };
   NSArray* constraints = @[
     @"V:[title]-(16)-[divider(==1)]|",
     @"H:|[v1(16)][title(<=440)][v2(>=v1)]|",
     @"H:|[divider]|",
   ];
   ApplyVisualConstraints(constraints, views);
-  return contentView = nil;
+  return contentView;
 }
 
 - (void)viewWillLayoutSubviews {
   CGSize viewSize = self.view.bounds.size;
   MDCFlexibleHeaderView* headerView =
       self.appBar.headerViewController.headerView;
   headerView.maximumHeight =
       MAX(kHeaderViewMinHeight, kHeaderViewHeightMultiplier * viewSize.height);
 }
 
@@ skipping 176 matching lines @@
       [self.collectionViewModel itemTypeForIndexPath:indexPath];
   return (itemType == ItemTypeTitle);
 }
 
 - (BOOL)collectionView:(nonnull UICollectionView*)collectionView
     shouldHideItemBackgroundAtIndexPath:(nonnull NSIndexPath*)indexPath {
   return YES;
 }
 
 @end