Remove JsonPrefStore pruning of empty values on write.

base/prefs/pref_registry.h

 // Copyright (c) 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 BASE_PREFS_PREF_REGISTRY_H_
 #define BASE_PREFS_PREF_REGISTRY_H_
 
-#include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/prefs/base_prefs_export.h"
 #include "base/prefs/pref_value_map.h"
 
 namespace base {
 class Value;
 }
 
 class DefaultPrefStore;
 class PrefStore;
 
 // Preferences need to be registered with a type and default value
 // before they are used.
 //
 // The way you use a PrefRegistry is that you register all required
 // preferences on it (via one of its subclasses), then pass it as a
 // construction parameter to PrefService.
 //
 // Currently, registrations after constructing the PrefService will
 // also work, but this is being deprecated.
 class BASE_PREFS_EXPORT PrefRegistry : public base::RefCounted<PrefRegistry> {
  public:
   typedef PrefValueMap::const_iterator const_iterator;
-  typedef base::Callback<void(const char*, base::Value*)> RegistrationCallback;
 
   PrefRegistry();
 
   // Gets the registered defaults.
   scoped_refptr<PrefStore> defaults();
 
   // Allows iteration over defaults.
   const_iterator begin() const;
   const_iterator end() const;
 
   // Changes the default value for a preference. Takes ownership of |value|.
   //
   // |pref_name| must be a previously registered preference.
   void SetDefaultPrefValue(const char* pref_name, base::Value* value);
 
-  // Exactly one callback can be set for registration. The callback
-  // will be invoked each time registration has been performed on this
-  // object.
-  //
-  // Calling this method after a callback has already been set will
-  // make the object forget the previous callback and use the new one
-  // instead.
-  void SetRegistrationCallback(const RegistrationCallback& callback);
-
  protected:
   friend class base::RefCounted<PrefRegistry>;
   virtual ~PrefRegistry();
 
   // Used by subclasses to register a default value for a preference.
   void RegisterPreference(const char* path, base::Value* default_value);
 
   scoped_refptr<DefaultPrefStore> defaults_;
 
  private:
-  RegistrationCallback registration_callback_;
-
   DISALLOW_COPY_AND_ASSIGN(PrefRegistry);
 };
 
 #endif  // BASE_PREFS_PREF_REGISTRY_H_