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/chrome/browser/web/sad_tab_tab_helper_delegate.h"
+#import "ios/web/public/web_state/web_state_observer.h"
+#import "ios/web/public/web_state/web_state_user_data.h"
+
+#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

[kkhorimoto, 2017/04/11 02:13:15]

Please finish comments with a period here and elsewhere in this CL.

[PL, 2017/04/11 03:08:56]

Done! (Maybe this is something we can have git cl format do in the future).

+class SadTabTabHelper : public web::WebStateUserData<SadTabTabHelper>,
+                        public web::WebStateObserver {
+ public:
+  // Create a SadTabTabHelper and attach it to a specific web_state object,
+  // an optional delegate object can be provided to control presentation
+  static void CreateForWebState(web::WebState* web_state,
+                                id<SadTabTabHelperDelegate> delegate);
+
+ private:
+  // Constructor for SadTabTabHelper, assigning the helper to a web_state and
+  // providing an optional delegate

[kkhorimoto, 2017/04/11 02:13:15]

Do we want the delegate to be optional?  I can't think of a reason why we would
want to show a SadTabView for a Tab that isn't currently visible....

[PL, 2017/04/11 03:08:56]

Done. This made more sense when I had the notion of a general delegate
interface.
This doesn't really have a reason now, so I've just taken it out.

+  SadTabTabHelper(web::WebState* web_state,
+                  id<SadTabTabHelperDelegate> delegate);
+
+  // Deconstructor for SadTabTabHelper
+  ~SadTabTabHelper() override;
+
+  // A TabHelperDelegate that can control aspects of this tab helper's behavior
+  __weak id<SadTabTabHelperDelegate> delegate_;
+
+  // Presents a new SadTabView via the web_state object
+  void PresentSadTab(const GURL& url_causing_failure);
+
+  // WebStateObserver:
+  void RenderProcessGone() override;
+};