[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/shared/chrome/browser/tabs/web_state_list_observer.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.
 
 #ifndef IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_
 
 #include "base/macros.h"
 
 class WebStateList;
@@ skipping 21 matching lines @@
                              int from_index,
                              int to_index);
 
   // Invoked after the WebState at the specified index is replaced by another
   // WebState.
   virtual void WebStateReplacedAt(WebStateList* web_state_list,
                                   web::WebState* old_web_state,
                                   web::WebState* new_web_state,
                                   int index);
 
+  // Invoked before the specified WebState is detached from the WebStateList.
+  // The WebState is still valid and still in the WebStateList.
+  virtual void WillDetachWebStateAt(WebStateList* web_state_list,
+                                    web::WebState* web_state,
+                                    int index);
+
   // Invoked after the WebState at the specified index has been detached. The
   // WebState is still valid but is no longer in the WebStateList.
   virtual void WebStateDetachedAt(WebStateList* web_state_list,
                                   web::WebState* web_state,
                                   int index);
 
+  // Invoked before the specified WebState is destroyed via the WebStateList.
+  // The WebState is still valid but is no longer in the WebStateList.
+  virtual void WillCloseWebStateAt(WebStateList* web_state_list,

[rohitrao (ping after 24h), 2017/03/24 01:20:52]

Why do we need this in addition to the existing
WebStateObserver::WebStateDestroyed() method?

Are these new methods going to be permanent, or are they only here to aid the
transition to WebStateList?

[rohitrao (ping after 24h), 2017/03/24 01:20:52]

I don't see this method being sent from anywhere.  Is that coming in a future
CL?

[sdefresne, 2017/03/24 09:26:18]

Yes, it is coming in a future CL
(https://codereview.chromium.org/2775623002/diff/20001/ios/shared/chrome/brows...).

It is different from WebStateObserver::WebStateDestroyed because it is called
before the destructor of WebState is called. The WebState destructor does some
internal cleanup before calling the observers (it close its CRWWebController)
and thus it is too late for some of the clients.

Finally, this method exists in TabStripModelObserver, it is called
TabStripModelObserver::TabClosingAt().

+                                   web::WebState* web_state,
+                                   int index);
+
   // Invoked after |new_web_state| was activated at the specified index. Both
   // WebState are either valid or null (if there was no selection or there is
   // no selection). If the change is due to an user action, |user_action| will
   // be true.
   virtual void WebStateActivatedAt(WebStateList* web_state_list,
                                    web::WebState* old_web_state,
                                    web::WebState* new_web_state,
                                    int active_index,
                                    bool user_action);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_