WebStateListObserver methods have default implementation.

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

Index: ios/shared/chrome/browser/tabs/web_state_list_observer.mm
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer.mm b/ios/shared/chrome/browser/tabs/web_state_list_observer.mm
new file mode 100644
index 0000000000000000000000000000000000000000..a5c510c2c21920af200d89d12b453a0529ea6fec
--- /dev/null
+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer.mm
@@ -0,0 +1,27 @@
+// 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_observer.h"
+
+WebStateListObserver::WebStateListObserver() = default;
+
+WebStateListObserver::~WebStateListObserver() = default;
+
+void WebStateListObserver::WebStateInsertedAt(WebStateList* web_state_list,
+                                              web::WebState* web_state,
+                                              int index) {}
+
+void WebStateListObserver::WebStateMoved(WebStateList* web_state_list,
+                                         web::WebState* web_state,
+                                         int from_index,
+                                         int to_index) {}
+
+void WebStateListObserver::WebStateReplacedAt(WebStateList* web_state_list,
+                                              web::WebState* old_web_state,
+                                              web::WebState* new_web_state,
+                                              int index) {}
+
+void WebStateListObserver::WebStateDetachedAt(WebStateList* web_state_list,
+                                              web::WebState* web_state,
+                                              int index) {}