Use a scoped WebStateListObserverBridge.

ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.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/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.h"
 
 #include <algorithm>
 #include <cstdint>
 #include <memory>
 
 #include "base/logging.h"
 #import "base/mac/foundation_util.h"
 #include "base/memory/ptr_util.h"
+#include "base/scoped_observer.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"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 @interface WebStateListFastEnumeration
     : NSObject<NSFastEnumeration, WebStateListObserving>
 
 - (instancetype)initWithWebStateList:(WebStateList*)webStateList
                         proxyFactory:(id<WebStateProxyFactory>)proxyFactory;
 
 - (void)shutdown;
 
 @end
 
 @implementation WebStateListFastEnumeration {
   // The wrapped WebStateList.
   WebStateList* _webStateList;
 
   // Helper that returns Objective-C proxies for WebState objects.
   id<WebStateProxyFactory> _proxyFactory;
 
   // WebStateListObserverBridge forwarding the events of WebStateList to self.
-  std::unique_ptr<WebStateListObserverBridge> _observerBridge;
+  std::unique_ptr<WebStateListObserverBridge> _webStateListObserver;
+  std::unique_ptr<ScopedObserver<WebStateList, WebStateListObserverBridge>>
+      _scopedWebStateListObserver;
 
   // Counter incremented each time the WebStateList is mutated.
   unsigned long _mutationCounter;
 }
 
 - (instancetype)initWithWebStateList:(WebStateList*)webStateList
                         proxyFactory:(id<WebStateProxyFactory>)proxyFactory {
   DCHECK(webStateList);
   DCHECK(proxyFactory);
   if ((self = [super init])) {
     _webStateList = webStateList;
     _proxyFactory = proxyFactory;
-    _observerBridge = base::MakeUnique<WebStateListObserverBridge>(self);
-    _webStateList->AddObserver(_observerBridge.get());
+    _webStateListObserver = base::MakeUnique<WebStateListObserverBridge>(self);
+    _scopedWebStateListObserver = base::MakeUnique<
+        ScopedObserver<WebStateList, WebStateListObserverBridge>>(
+        _webStateListObserver.get());
+    _scopedWebStateListObserver->Add(_webStateList);
   }
   return self;
 }
 
 - (void)shutdown {
-  _webStateList->RemoveObserver(_observerBridge.get());

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

This is incorrect, the registration needs to happens when the -shutdown method
is called. The correct solution with a ScopedObserver would be to delete the
ScopedObserver at that point, but I think it does not bring enough value since
this class need to explicitly control when registration/unregistration happen
anyway.

I would suggest reverting this file and leaving it to use manual
AddObserver/RemoveObserver calls.

BTW, this is because of this that all tests are failing.

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

Done.

   _webStateList = nullptr;
   ++_mutationCounter;
 }
 
 - (void)dealloc {
   DCHECK(!_webStateList) << "-shutdown must be called before -dealloc";
 }
 
 #pragma mark NSFastEnumeration
 
@@ skipping 71 matching lines @@
   WebStateListFastEnumeration* fast_enumeration =
       base::mac::ObjCCastStrict<WebStateListFastEnumeration>(
           fast_enumeration_.get());
   [fast_enumeration shutdown];
   fast_enumeration_.reset();
 }
 
 id<NSFastEnumeration> WebStateListFastEnumerationHelper::GetFastEnumeration() {
   return fast_enumeration_.get();
 }