Refactor creation of SadTabView into a tab helper object

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:
+  // 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(const GURL& url_causing_failure);
+
+  // WebStateObserver:
+  void RenderProcessGone() override;
+
+  // A TabHelperDelegate that can control aspects of this tab helper's behavior.
+  __weak id<SadTabTabHelperDelegate> delegate_;
+};