WebStateListObserver methods have default implementation.

ios/shared/chrome/browser/tabs/web_state_list_observer.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_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_
 
 #include "base/macros.h"
 
 class WebStateList;
 
 namespace web {
 class WebState;
 }
 
 // Interface for listening to events occurring to WebStateLists.
 class WebStateListObserver {
  public:
-  WebStateListObserver() = default;
-  virtual ~WebStateListObserver() = default;
+  WebStateListObserver();
+  virtual ~WebStateListObserver();
 
   // Invoked after a new WebState has been added to the WebStateList at the
   // specified index.
   virtual void WebStateInsertedAt(WebStateList* web_state_list,
                                   web::WebState* web_state,
-                                  int index) = 0;
+                                  int index);
 
   // Invoked after the WebState at the specified index is moved to another
   // index.
   virtual void WebStateMoved(WebStateList* web_state_list,
                              web::WebState* web_state,
                              int from_index,
-                             int to_index) = 0;
+                             int to_index);
 
   // Invoked after the WebState at the specified index is replaced by another
   // WebState.
   virtual void WebStateReplacedAt(WebStateList* web_state_list,
                                   web::WebState* old_web_state,
                                   web::WebState* new_web_state,
-                                  int index) = 0;
+                                  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.
   virtual void WebStateDetachedAt(WebStateList* web_state_list,
                                   web::WebState* web_state,
-                                  int index) = 0;
+                                  int index);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_