Switch SupportsUserData uses to use unique_ptr.

ios/chrome/browser/web/sad_tab_tab_helper.h

 // Copyright 2017 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/web/public/web_state/web_state_observer.h"
 #import "ios/web/public/web_state/web_state_user_data.h"
 
 @protocol SadTabTabHelperDelegate;
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 // SadTabTabHelper listens to RenderProcessGone events and presents a
 // SadTabView view appropriately.
 class SadTabTabHelper : public web::WebStateUserData<SadTabTabHelper>,
                         public web::WebStateObserver {
  public:
+  ~SadTabTabHelper() override;
+
   // Creates a SadTabTabHelper and attaches it to a specific web_state object,
   // a delegate object controls presentation.
   static void CreateForWebState(web::WebState* web_state,
                                 id<SadTabTabHelperDelegate> delegate);
 
  private:
   // Constructor for SadTabTabHelper, assigning the helper to a web_state and
   // providing a delegate.
   SadTabTabHelper(web::WebState* web_state,
                   id<SadTabTabHelperDelegate> delegate);
 
-  ~SadTabTabHelper() override;
-
   // Presents a new SadTabView via the web_state object.
   void PresentSadTab();
 
   // WebStateObserver:
   void RenderProcessGone() override;
 
   // A TabHelperDelegate that can control aspects of this tab helper's behavior.
   __weak id<SadTabTabHelperDelegate> delegate_;
 };