Refactor creation of SadTabView into a tab helper object

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
new file mode 100644
index 0000000000000000000000000000000000000000..051d4e858248e7fa807a8a06485ac7348ce3f9c4
--- /dev/null
+++ b/ios/chrome/browser/web/sad_tab_tab_helper.h
@@ -0,0 +1,43 @@
+// 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"

[Eugene But (OOO till 7-30), 2017/04/11 15:17:00]

nit: Would predeclaration be sufficient here?

C++ Guides advocate for includes and Chromium Style Guide for predeclarations.
Chromium Style Guide overrides C++ Style Guide.

[peterlaurens, 2017/04/11 21:57:26]

Yes! Done, thanks!

+#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.
+class SadTabTabHelper : public web::WebStateUserData<SadTabTabHelper>,
+                        public web::WebStateObserver {
+ public:
+  // Create a SadTabTabHelper and attach it to a specific web_state object,

[Eugene But (OOO till 7-30), 2017/04/11 15:17:00]

s/Create/Creates s/attach/attaches

[peterlaurens, 2017/04/11 21:57:26]

Done, thanks.

+  // 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 a delegate.
+  SadTabTabHelper(web::WebState* web_state,
+                  id<SadTabTabHelperDelegate> delegate);
+
+  // WebStateUserData:
+  explicit SadTabTabHelper(web::WebState* web_state);

[sdefresne, 2017/04/11 08:17:39]

You should be able to remove this constructor.

Note that if you remove the constructor, calling
"SadTabTabHelper::CreateForWebState(web_state)" will cause a compilation error
(constructor does not exists) which is better than a runtime error (NOTREACHED).

[kkhorimoto, 2017/04/11 17:38:02]

Are you sure it would cause a compilation error?  I would assume that it would
just call the superclass's constructor and leave |delegate_| uninitialized in
this subclass, which is why I suggested keeping this constructor and adding a
NOTREACHED.  Peter, can you verify whether calling
SadTabTabHelper::CreateForWebState(WebState*) causes a compilation error and
remove if it does?  Sorry for the back and forth on this!

[peterlaurens, 2017/04/11 21:57:27]

Done. Can confirm that a compilation error occurs (even if including headers
from the 
superclasses). I'll take this out, thanks!

+
+  // Deconstructor for SadTabTabHelper.

[Eugene But (OOO till 7-30), 2017/04/11 15:17:00]

nit: do we need this comment? Technically it is "Destructor", but that's quite
obvious from the code.

[peterlaurens, 2017/04/11 21:57:26]

Done.

+  ~SadTabTabHelper() override;
+
+  // A TabHelperDelegate that can control aspects of this tab helper's behavior.
+  __weak id<SadTabTabHelperDelegate> delegate_;

[Eugene But (OOO till 7-30), 2017/04/11 15:17:00]

Could you please move data member to the bottom (after methods).

https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.shtml?cl=head#...

[peterlaurens, 2017/04/11 21:57:26]

Done, thanks!

+
+  // Presents a new SadTabView via the web_state object.
+  void PresentSadTab(const GURL& url_causing_failure);
+
+  // WebStateObserver:
+  void RenderProcessGone() override;
+};