Fix WebStateListObserver::WebStateDetachedAt() parameter lifetime.

ios/shared/chrome/browser/tabs/web_state_list.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_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_H_
 
 #include <memory>
 #include <vector>
 
+#include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "ui/base/page_transition_types.h"
 
 class WebStateListObserver;
 class WebStateListOrderController;
 
 namespace web {
 class WebState;
 }
@@ skipping 66 matching lines @@
                       web::WebState* opener);
 
   // Moves the WebState at the specified index to another index.
   void MoveWebStateAt(int from_index, int to_index);
 
   // Replaces the WebState at the specified index with new WebState. Returns
   // the old WebState at that index to the caller (abandon ownership of the
   // returned WebState). An optional opener for the new WebState may be passed.
   web::WebState* ReplaceWebStateAt(int index,
                                    web::WebState* web_state,
-                                   web::WebState* opener);
+                                   web::WebState* opener) WARN_UNUSED_RESULT;
 
-  // Detaches the WebState at the specified index.
-  void DetachWebStateAt(int index);
+  // Detaches the WebState at the specified index. Returns the detached WebState
+  // to the caller (abandon ownership of the returned WebState).
+  web::WebState* DetachWebStateAt(int index) WARN_UNUSED_RESULT;
 
   // Adds an observer to the model.
   void AddObserver(WebStateListObserver* observer);
 
   // Removes an observer from the model.
   void RemoveObserver(WebStateListObserver* observer);
 
   // Invalid index.
   static const int kInvalidIndex = -1;
 
@@ skipping 20 matching lines @@
   // selection should move when a WebState is detached.
   std::unique_ptr<WebStateListOrderController> order_controller_;
 
   // List of observers notified of changes to the model.
   base::ObserverList<WebStateListObserver, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WebStateList);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_H_