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

ios/chrome/browser/prefs/pref_observer_bridge.h

 // Copyright 2012 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_PREFS_PREF_OBSERVER_BRIDGE_H_
 #define IOS_CHROME_BROWSER_PREFS_PREF_OBSERVER_BRIDGE_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 
 class PrefChangeRegistrar;
 
 @protocol PrefObserverDelegate
 - (void)onPreferenceChanged:(const std::string&)preferenceName;
 @end
 
 class PrefObserverBridge {
  public:
   explicit PrefObserverBridge(id<PrefObserverDelegate> delegate);
   virtual ~PrefObserverBridge();
 
   virtual void ObserveChangesForPreference(const std::string& pref_name,
                                            PrefChangeRegistrar* registrar);
 
  private:
   virtual void OnPreferenceChanged(const std::string& pref_name);
 
-  id<PrefObserverDelegate> delegate_;
+  __unsafe_unretained id<PrefObserverDelegate> delegate_;
 };
 
 #endif  // IOS_CHROME_BROWSER_PREFS_PREF_OBSERVER_BRIDGE_H_