[ObjC ARC] Converts ios/chrome/browser/ui/bookmarks:bookmarks to ARC.

ios/chrome/browser/ui/bookmarks/undo_manager_bridge_observer.h

 // Copyright 2014 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.
 
 #ifndef IOS_CHROME_BROWSER_UI_BOOKMARKS_UNDO_MANAGER_BRIDGE_OBSERVER_H_
 #define IOS_CHROME_BROWSER_UI_BOOKMARKS_UNDO_MANAGER_BRIDGE_OBSERVER_H_
 
 #include "base/compiler_specific.h"
 #include "components/undo/undo_manager_observer.h"
 
 // The ObjC translations of the C++ observer callbacks are defined here.
 @protocol UndoManagerBridgeObserver
 // Invoked when the internal state of the undo manager has changed.
 - (void)undoManagerChanged;
 @end
 
 namespace bookmarks {
 // A bridge that translates UndoManagerObserver C++ callbacks into ObjC
 // callbacks.
 class UndoManagerBridge : public UndoManagerObserver {
  public:
   explicit UndoManagerBridge(id<UndoManagerBridgeObserver> observer);
   ~UndoManagerBridge() override{};
 
  private:
   void OnUndoManagerStateChange() override;
-  id<UndoManagerBridgeObserver> observer_;
+  __weak id<UndoManagerBridgeObserver> observer_;
 };
 }  // namespace bookmarks
 
 #endif  // IOS_CHROME_BROWSER_UI_BOOKMARKS_UNDO_MANAGER_BRIDGE_OBSERVER_H_