Move the notion of current Tab from TabModel to WebStateList.

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

Index: ios/shared/chrome/browser/tabs/web_state_list_observer.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer.h b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
index bbe3c4005a8f84f5715854397047c95938dd9c98..4c725fb24a8ff5776fb61fa1a3c1767671875cd2 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_observer.h
+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
@@ -45,6 +45,16 @@ class WebStateListObserver {
                                   web::WebState* web_state,
                                   int index);
 
+  // Invoked after |new_web_state| 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, |user_action| will
+  // be true.
+  virtual void WebStateActivatedAt(WebStateList* web_state_list,
+                                   web::WebState* old_web_state,
+                                   web::WebState* new_web_state,
+                                   int active_index,
+                                   bool user_action);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
 };