Use a scoped WebStateListObserverBridge.

ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm

Index: ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm
diff --git a/ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm b/ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm
index b2e727b3b5f377ef91c2f34801e85eaf1d1d90c7..1c07849dc29bbf90b2e312e9a829eae677bb830f 100644
--- a/ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm
+++ b/ios/chrome/browser/web_state_list/web_state_list_fast_enumeration_helper.mm
@@ -11,6 +11,7 @@
 #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"
 
@@ -36,7 +37,9 @@
   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;
@@ -49,14 +52,16 @@
   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;
 }