Remove -insertTabWithWebState:atIndex: from TabModel.

ios/chrome/browser/tabs/tab_model_observers_bridge.mm

Index: ios/chrome/browser/tabs/tab_model_observers_bridge.mm
diff --git a/ios/chrome/browser/tabs/tab_model_observers_bridge.mm b/ios/chrome/browser/tabs/tab_model_observers_bridge.mm
index 780f14739dac169e11625af34ac41f41b710bf1a..df7be5f10cb365ea4928168c57798085d5734600 100644
--- a/ios/chrome/browser/tabs/tab_model_observers_bridge.mm
+++ b/ios/chrome/browser/tabs/tab_model_observers_bridge.mm
@@ -80,10 +80,14 @@
                 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