Use a scoped WebStateListObserverBridge.

ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm

Index: ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm
diff --git a/ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm b/ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm
index 1ff70c0ea6e5fb63a75394fc04e1e802c5b9502c..99ccc909045783ea1aac6bb6d5174029316e00b9 100644
--- a/ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm
+++ b/ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm
@@ -5,6 +5,7 @@
 #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"
@@ -29,6 +30,8 @@
   // 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
@@ -54,7 +57,10 @@
     _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 =
@@ -69,10 +75,6 @@
   return self;
 }
 
-- (void)dealloc {
-  _webStateList->RemoveObserver(_webStateListObserver.get());
-}
-
 - (void)setLocationBar:(std::unique_ptr<LocationBarController>)locationBar {
   _locationBar = std::move(locationBar);
   _locationBar->OnToolbarUpdated();