Use a scoped WebStateListObserverBridge.

ios/clean/chrome/browser/ui/tab_collection/tab_collection_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/tab_collection/tab_collection_mediator.h"
 
 #include "base/memory/ptr_util.h"
+#include "base/scoped_observer.h"
 #include "base/strings/sys_string_conversions.h"
 #import "ios/chrome/browser/web_state_list/web_state_list.h"
 #import "ios/clean/chrome/browser/ui/tab_collection/tab_collection_consumer.h"
 #include "ios/web/public/web_state/web_state.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 @implementation TabCollectionMediator {
   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 {
   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]

You can use RemoveAll/Add to avoid destroying the observer and ScopedObserver
everytime.

- (instancetype)init {
  self = [super init];
  if (self) {
    _webStateListObserver = base::MakeUnique<WebStateListObserverBridge>(self);
    _scopedWebStateListObserver = base::MakeUnique<
        ScopedObserver<WebStateList, WebStateListObserverBridge>>(
        _webStateListObserver.get());
  }
}

- (void)setWebStateList:(WebStateList*)webStateList {
  _scopedWebStateListObserver->RemoveAll();
  _webStateList = webStateList;
  if (!_webStateList)
    return;
  _scopedWebStateListObserver->Add(_webStateList);
}

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

Done, although could we imagine a time when RemoveAll removes too much?

+  _webStateListObserver.reset();
   _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);
 }
 
 #pragma mark - TabCollectionDataSource
 
 - (int)numberOfTabs {
   return self.webStateList->count();
 }
 
 - (NSString*)titleAtIndex:(int)index {
   return [self titleFromWebState:self.webStateList->GetWebStateAt(index)];
@@ skipping 57 matching lines @@
   // PLACEHOLDER: Use real webstate title in the future.
   GURL url = webState->GetVisibleURL();
   NSString* urlText = @"<New Tab>";
   if (url.is_valid()) {
     urlText = base::SysUTF8ToNSString(url.spec());
   }
   return urlText;
 }
 
 @end