[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/shared/chrome/browser/tabs/web_state_list_observer.h

Index: ios/shared/chrome/browser/tabs/web_state_list_observer.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer.h b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
index 4c725fb24a8ff5776fb61fa1a3c1767671875cd2..4d4134162f0962d7970e212e11c32ba7bf9e4b13 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_observer.h
+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
@@ -39,12 +39,24 @@ class WebStateListObserver {
                                   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,
+                                   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