Move the notion of current Tab from TabModel to WebStateList.

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

 // 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.
 
 #ifndef IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_ORDER_CONTROLLER_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_ORDER_CONTROLLER_H_
 
 #include "base/macros.h"
 #include "ui/base/page_transition_types.h"
 
 class WebStateList;
 
 namespace web {
 class WebState;
 }
 
 // WebStateListOrderController allows different types of ordering and
 // selection heuristics to be plugged into WebStateList.
 class WebStateListOrderController {
  public:
   explicit WebStateListOrderController(WebStateList* web_state_list);
   ~WebStateListOrderController();
 
   // Determines where to place a newly opened WebState by using the supplied
   // transition, opener and background flag.
   int DetermineInsertionIndex(ui::PageTransition transition,
                               web::WebState* opener) const;
 
+  // Determines where to shift the active index after a WebState is closed.
+  int DetermineNewActiveIndex(int removing_index) const;
+
  private:
+  // Returns a valid index to be selected after the WebState at |removing_index|
+  // is detached, adjusting |index| to reflect that |removing_index| is going
+  // away.
+  int GetValidIndex(int index, int removing_index) const;
+
   WebStateList* web_state_list_;
 
   DISALLOW_COPY_AND_ASSIGN(WebStateListOrderController);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_ORDER_CONTROLLER_H_