Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.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_OBSERVER_BRIDGE_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_BRIDGE_H_
 
 #import <Foundation/Foundation.h>
 
 #include "base/macros.h"
@@ skipping 23 matching lines @@
     didReplaceWebState:(web::WebState*)oldWebState
           withWebState:(web::WebState*)newWebState
                atIndex:(int)index;
 
 // Invoked after the WebState at the specified index has been detached. The
 // WebState is still valid but is no longer in the WebStateList.
 - (void)webStateList:(WebStateList*)webStateList
     didDetachWebState:(web::WebState*)webState
               atIndex:(int)index;
 
+// Invoked after |newWebState| was activated at the specified index. Both
+// WebState are either valid or null (if there was no selection or there is
+// no selection). If the change is due to an user action, |userAction| will
+// be true.
+- (void)webStateList:(WebStateList*)webStateList
+    didChangeActiveWebState:(web::WebState*)newWebState
+                oldWebState:(web::WebState*)oldWebState
+                    atIndex:(int)atIndex
+                 userAction:(BOOL)userAction;
+
 @end
 
 // Observer that bridges WebStateList events to an Objective-C observer that
 // implements the WebStateListObserver protocol (the observer is owned).
 class WebStateListObserverBridge : public WebStateListObserver {
  public:
   explicit WebStateListObserverBridge(id<WebStateListObserving> observer);
   ~WebStateListObserverBridge() override;
 
  private:
   // WebStateListObserver implementation.
   void WebStateInsertedAt(WebStateList* web_state_list,
                           web::WebState* web_state,
                           int index) override;
   void WebStateMoved(WebStateList* web_state_list,
                      web::WebState* web_state,
                      int from_index,
                      int to_index) override;
   void WebStateReplacedAt(WebStateList* web_state_list,
                           web::WebState* old_web_state,
                           web::WebState* new_web_state,
                           int index) override;
   void WebStateDetachedAt(WebStateList* web_state_list,
                           web::WebState* web_state,
                           int index) override;
+  void WebStateActivatedAt(WebStateList* web_state_list,
+                           web::WebState* old_web_state,
+                           web::WebState* new_web_state,
+                           int active_index,
+                           bool user_action) override;
 
   id<WebStateListObserving> observer_;
 
   DISALLOW_COPY_AND_ASSIGN(WebStateListObserverBridge);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_BRIDGE_H_