Use a scoped WebStateListObserverBridge.

ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.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/clean/chrome/browser/ui/omnibox/location_bar_mediator.h"
 
 #include "base/memory/ptr_util.h"
+#include "base/scoped_observer.h"
 #include "base/strings/utf_string_conversions.h"
 #include "ios/chrome/browser/ui/toolbar/toolbar_model_delegate_ios.h"
 #include "ios/chrome/browser/ui/toolbar/toolbar_model_impl_ios.h"
 #include "ios/chrome/browser/web_state_list/web_state_list.h"
 #include "ios/chrome/browser/web_state_list/web_state_list_observer_bridge.h"
 #import "ios/shared/chrome/browser/ui/omnibox/location_bar_controller.h"
 #import "ios/web/public/navigation_manager.h"
 #import "ios/web/public/web_state/web_state.h"
 #import "ios/web/public/web_state/web_state_observer_bridge.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 @interface LocationBarMediator ()<CRWWebStateObserver,
                                   LocationBarDelegate,
                                   WebStateListObserving>
 @property(nonatomic, readwrite, assign) WebStateList* webStateList;
 @end
 
 @implementation LocationBarMediator {
   // Observes the WebStateList so that this mediator can update the UI when the
   // active WebState changes.
   std::unique_ptr<WebStateListObserverBridge> _webStateListObserver;
+  std::unique_ptr<ScopedObserver<WebStateList, WebStateListObserverBridge>>
+      _scopedWebStateListObserver;
 
   // Used to update the UI in response to WebState observer notifications.  This
   // observer is always observing the currently-active WebState and may be
   // nullptr if no WebState is currently active.
   std::unique_ptr<web::WebStateObserverBridge> _webStateObserver;
 
   // The LocationBarController that wraps OmniboxViewIOS and
   // OmniboxTextFieldIOS.  This mediator updates the UI through |_locationBar|
   // rather than through a consumer.
   std::unique_ptr<LocationBarController> _locationBar;
 
   // The ToolbarModelDelegate, backed by |_webStateList|.
   std::unique_ptr<ToolbarModelDelegateIOS> _toolbarModelDelegate;
 
   // The ToolbarModel that backs |_locationBar|.
   std::unique_ptr<ToolbarModelImplIOS> _toolbarModel;
 }
 
 @synthesize webStateList = _webStateList;
 
 - (instancetype)initWithWebStateList:(WebStateList*)webStateList {
   if ((self = [super init])) {
     _webStateList = webStateList;
 
     _webStateListObserver = base::MakeUnique<WebStateListObserverBridge>(self);
-    _webStateList->AddObserver(_webStateListObserver.get());
+    _scopedWebStateListObserver = base::MakeUnique<
+        ScopedObserver<WebStateList, WebStateListObserverBridge>>(
+        _webStateListObserver.get());
+    _scopedWebStateListObserver->Add(_webStateList);
     web::WebState* webState = _webStateList->GetActiveWebState();
     if (webState) {
       _webStateObserver =
           base::MakeUnique<web::WebStateObserverBridge>(webState, self);
     }
 
     _toolbarModelDelegate =
         base::MakeUnique<ToolbarModelDelegateIOS>(webStateList);
     _toolbarModel =
         base::MakeUnique<ToolbarModelImplIOS>(_toolbarModelDelegate.get());
   }
   return self;
 }
 
-- (void)dealloc {
-  _webStateList->RemoveObserver(_webStateListObserver.get());
-}
-
 - (void)setLocationBar:(std::unique_ptr<LocationBarController>)locationBar {
   _locationBar = std::move(locationBar);
   _locationBar->OnToolbarUpdated();
 }
 
 #pragma mark - LocationBarDelegate
 
 - (void)loadGURLFromLocationBar:(const GURL&)url
                      transition:(ui::PageTransition)transition {
   web::WebState* webState = _webStateList->GetActiveWebState();
@@ skipping 96 matching lines @@
 
 - (void)webStateDidChangeVisibleSecurityState:(web::WebState*)webState {
   _locationBar->OnToolbarUpdated();
 }
 
 - (void)webStateDestroyed:(web::WebState*)webState {
   _webStateObserver = nullptr;
 }
 
 @end