[clean ios] Fix TabCollectionMediator disconnect.

ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm

 // 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/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/scoped_observer.h"
 #include "base/strings/sys_string_conversions.h"
 #import "ios/chrome/browser/web_state_list/web_state_list.h"
@@ skipping 32 matching lines @@
 
 - (void)dealloc {
   [self disconnect];
 }
 
 #pragma mark - Public
 
 - (void)disconnect {
   _webStateList = nullptr;
   _webStateObserver.reset();
+  _scopedWebStateListObserver->RemoveAll();
 }
 
 #pragma mark - Properties
 
 - (void)setWebStateList:(WebStateList*)webStateList {
   DCHECK(webStateList);
   _scopedWebStateListObserver->RemoveAll();
   _webStateList = webStateList;
   _scopedWebStateListObserver->Add(_webStateList);
   _webStateObserver = base::MakeUnique<web::WebStateObserverBridge>(
@@ skipping 104 matching lines @@
 - (void)populateConsumerItems {
   if (self.consumer && self.webStateList) {
     [self.consumer
         populateItems:[self
                           tabCollectionItemsFromWebStateList:self.webStateList]
         selectedIndex:self.webStateList->active_index()];
   }
 }
 
 @end