WebStateListObserver methods have default implementation.

ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.mm

 // 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.
 
 #import "ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.h"
 
 #include "base/metrics/user_metrics.h"
 #include "base/metrics/user_metrics_action.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 WebStateListMetricsObserver::WebStateListMetricsObserver() = default;
 
 WebStateListMetricsObserver::~WebStateListMetricsObserver() = default;
 
 void WebStateListMetricsObserver::WebStateInsertedAt(
     WebStateList* web_state_list,
     web::WebState* web_state,
     int index) {
   base::RecordAction(base::UserMetricsAction("MobileNewTabOpened"));
 }
 
-void WebStateListMetricsObserver::WebStateMoved(WebStateList* web_state_list,
-                                                web::WebState* web_state,
-                                                int from_index,
-                                                int to_index) {}
-
 void WebStateListMetricsObserver::WebStateReplacedAt(
     WebStateList* web_state_list,
     web::WebState* old_web_state,
     web::WebState* new_web_state,
     int index) {
   // Record a tab clobber, since swapping tabs bypasses the Tab code that would
   // normally log clobbers.
   base::RecordAction(base::UserMetricsAction("MobileTabClobbered"));
 }
 
 void WebStateListMetricsObserver::WebStateDetachedAt(
     WebStateList* web_state_list,
     web::WebState* web_state,
     int index) {
   base::RecordAction(base::UserMetricsAction("MobileTabClosed"));
 }