Use a scoped WebStateListObserverBridge.

ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm

Index: ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm
diff --git a/ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm b/ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm
index dcf98cfadba86dd395bae0f67cedaeeabbb9d66a..3ca3b7af4295698d82fafc2472a5af3990d8d919 100644
--- a/ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm
+++ b/ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.mm
@@ -5,6 +5,7 @@
 #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"
@@ -16,6 +17,8 @@
 
 @implementation TabCollectionMediator {
   std::unique_ptr<WebStateListObserverBridge> _webStateListObserver;
+  std::unique_ptr<ScopedObserver<WebStateList, WebStateListObserverBridge>>
+      _scopedWebStateListObserver;
 }
 
 @synthesize webStateList = _webStateList;
@@ -34,16 +37,17 @@
 #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