Switch SupportsUserData uses to use unique_ptr.

ios/chrome/browser/web/sad_tab_tab_helper.h

Index: ios/chrome/browser/web/sad_tab_tab_helper.h
diff --git a/ios/chrome/browser/web/sad_tab_tab_helper.h b/ios/chrome/browser/web/sad_tab_tab_helper.h
index 838f5c6384347675f472c313437d61c2ac4905c5..25240becf41bed83f3dc7ef57bd9d0d75405b5f0 100644
--- a/ios/chrome/browser/web/sad_tab_tab_helper.h
+++ b/ios/chrome/browser/web/sad_tab_tab_helper.h
@@ -16,6 +16,8 @@
 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,
@@ -27,8 +29,6 @@ class SadTabTabHelper : public web::WebStateUserData<SadTabTabHelper>,
   SadTabTabHelper(web::WebState* web_state,
                   id<SadTabTabHelperDelegate> delegate);
 
-  ~SadTabTabHelper() override;
-
   // Presents a new SadTabView via the web_state object.
   void PresentSadTab();