Use a scoped WebStateListObserverBridge.

ios/clean/chrome/browser/ui/web_contents/web_contents_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/web_contents/web_contents_mediator.h"
 
 #include "base/memory/ptr_util.h"
+#include "base/scoped_observer.h"
 #include "ios/chrome/browser/chrome_url_constants.h"
 #import "ios/chrome/browser/web_state_list/web_state_list.h"
 #import "ios/chrome/browser/web_state_list/web_state_list_observer_bridge.h"
 #import "ios/clean/chrome/browser/ui/web_contents/web_contents_consumer.h"
 #import "ios/web/public/navigation_manager.h"
 #include "ios/web/public/web_state/web_state.h"
 #include "ui/base/page_transition_types.h"
 #include "url/gurl.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 @interface WebContentsMediator ()<WebStateListObserving>
 @end
 
 @implementation WebContentsMediator {
   std::unique_ptr<WebStateListObserverBridge> _webStateListObserver;
+  std::unique_ptr<ScopedObserver<WebStateList, WebStateListObserverBridge>>
+      _scopedWebStateListObserver;
 }
 @synthesize webStateList = _webStateList;
 @synthesize consumer = _consumer;
 
 - (void)dealloc {
   [self disconnect];
 }
 
 #pragma mark - Public
 
 - (void)disconnect {
   if (!self.webStateList) {
     return;
   }
   [self disableWebUsage:self.webStateList->GetActiveWebState()];
   self.webStateList = nullptr;
 }
 
 #pragma mark - Properties
 
 - (void)setWebStateList:(WebStateList*)webStateList {
-  if (_webStateList) {
-    _webStateList->RemoveObserver(_webStateListObserver.get());
-    _webStateListObserver.reset();
-  }
+  _scopedWebStateListObserver.reset();

[sdefresne, 2017/04/20 10:02:07]

ditto, use RemoveAll/Add to avoid creating destroying the observer and
ScopedObserver everytime

[lpromero, 2017/04/20 16:25:13]

Done.

+  _webStateListObserver.reset();

[sdefresne, 2017/04/20 10:02:07]

Why don't we call "[self
disableWebUsage:self.webStateList->GetActiveWebState()];" here?

[lpromero, 2017/04/20 16:25:13]

No idea. I filed http://crbug.com/713757.

   _webStateList = webStateList;
   if (!_webStateList) {
     return;
   }
   _webStateListObserver = base::MakeUnique<WebStateListObserverBridge>(self);
-  _webStateList->AddObserver(_webStateListObserver.get());
+  _scopedWebStateListObserver = base::MakeUnique<
+      ScopedObserver<WebStateList, WebStateListObserverBridge>>(
+      _webStateListObserver.get());
+  _scopedWebStateListObserver->Add(_webStateList);
   if (_webStateList->GetActiveWebState()) {
     [self updateConsumerWithWebState:_webStateList->GetActiveWebState()];
   }
 }
 
 - (void)setConsumer:(id<WebContentsConsumer>)consumer {
   _consumer = consumer;
   if (self.webStateList && self.webStateList->GetActiveWebState()) {
     [self updateConsumerWithWebState:self.webStateList->GetActiveWebState()];
   }
@@ skipping 36 matching lines @@
 // PLACEHOLDER: This navigates an empty webstate to the NTP.
 - (void)navigateToDefaultPage:(web::WebState*)webState {
   if (!webState->GetNavigationManager()->GetItemCount()) {
     web::NavigationManager::WebLoadParams params((GURL(kChromeUINewTabURL)));
     params.transition_type = ui::PAGE_TRANSITION_TYPED;
     webState->GetNavigationManager()->LoadURLWithParams(params);
   }
 }
 
 @end