Move the notion of current Tab from TabModel to WebStateList.

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

Index: ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h b/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h
index 3cfbe2da7394ae0f74444167ac1277bf6713a4c6..e412fb288dd13cd99c2d34959ebe4636d16ffc58 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h
+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h
@@ -41,6 +41,16 @@
     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
@@ -66,6 +76,11 @@ class WebStateListObserverBridge : public WebStateListObserver {
   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_;