[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/shared/chrome/browser/tabs/web_state_list.mm

Index: ios/shared/chrome/browser/tabs/web_state_list.mm
diff --git a/ios/shared/chrome/browser/tabs/web_state_list.mm b/ios/shared/chrome/browser/tabs/web_state_list.mm
index c3614afa144465d94615e780bf5a5986a88e18c8..1ae68ba034f2b5addd39ed4475dc6f345c51b289 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list.mm
+++ b/ios/shared/chrome/browser/tabs/web_state_list.mm
@@ -214,6 +214,7 @@ web::WebState* WebStateList::ReplaceWebStateAt(int index,
   for (auto& observer : observers_)
     observer.WebStateReplacedAt(this, old_web_state, web_state, index);
 
+  delegate_->WebStateDetached(old_web_state);
   return old_web_state;
 }
 
@@ -222,6 +223,8 @@ web::WebState* WebStateList::DetachWebStateAt(int index) {
   int new_active_index = order_controller_->DetermineNewActiveIndex(index);
 
   web::WebState* old_web_state = web_state_wrappers_[index]->web_state();
+  for (auto& observer : observers_)
+    observer.WillDetachWebStateAt(this, old_web_state, index);
 
   ClearOpenersReferencing(index);
   web_state_wrappers_.erase(web_state_wrappers_.begin() + index);
@@ -241,6 +244,7 @@ web::WebState* WebStateList::DetachWebStateAt(int index) {
   if (active_web_state_was_closed)
     NotifyIfActiveWebStateChanged(old_web_state, false);
 
+  delegate_->WebStateDetached(old_web_state);
   return old_web_state;
 }