Remove -insertTabWithWebState:atIndex: from TabModel.

ios/chrome/browser/tabs/tab_model_observers_bridge.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/chrome/browser/tabs/tab_model_observers_bridge.h"
 
 #include "base/logging.h"
 #import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 #import "ios/chrome/browser/tabs/tab_model.h"
 #import "ios/chrome/browser/tabs/tab_model_observers.h"
@@ skipping 62 matching lines @@
                   didRemoveTab:LegacyTabHelper::GetTabForWebState(webState)
                        atIndex:static_cast<NSUInteger>(index)];
   [_tabModelObservers tabModelDidChangeTabCount:_tabModel];
 }
 
 - (void)webStateList:(WebStateList*)webStateList
     didChangeActiveWebState:(web::WebState*)newWebState
                 oldWebState:(web::WebState*)oldWebState
                     atIndex:(int)atIndex
                  userAction:(BOOL)userAction {
-  DCHECK_GE(atIndex, 0);
   if (!newWebState)
     return;
 
+  // If there is no new active WebState, then it means that the atIndex will be
+  // set to WebStateList::kInvalidIndex, so only check for a positive index if
+  // there is a new WebState.
+  DCHECK_GE(atIndex, 0);
+
   Tab* oldTab =
       oldWebState ? LegacyTabHelper::GetTabForWebState(oldWebState) : nil;
   [_tabModelObservers tabModel:_tabModel
             didChangeActiveTab:LegacyTabHelper::GetTabForWebState(newWebState)
                    previousTab:oldTab
                        atIndex:static_cast<NSUInteger>(atIndex)];
 }
 
 @end