Fix a race condition in preference metric reporting.

base/prefs/pref_store.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_STORE_H_
 #define BASE_PREFS_PREF_STORE_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/prefs/base_prefs_export.h"
 
 namespace base {
 class Value;
 }
 
 // This is an abstract interface for reading and writing from/to a persistent
 // preference store, used by PrefService. An implementation using a JSON file
 // can be found in JsonPrefStore, while an implementation without any backing
 // store for testing can be found in TestingPrefStore. Furthermore, there is
 // CommandLinePrefStore, which bridges command line options to preferences and
 // ConfigurationPolicyPrefStore, which is used for hooking up configuration
 // policy with the preference subsystem.
 class BASE_PREFS_EXPORT PrefStore : public base::RefCounted<PrefStore> {
  public:
   // Observer interface for monitoring PrefStore.
   class BASE_PREFS_EXPORT Observer {
    public:
+    virtual ~Observer() {}
+
     // Called when the value for the given |key| in the store changes.
     virtual void OnPrefValueChanged(const std::string& key) = 0;
     // Notification about the PrefStore being fully initialized.
     virtual void OnInitializationCompleted(bool succeeded) = 0;
-
-   protected:
-    virtual ~Observer() {}
   };
 
   PrefStore() {}
 
   // Add and remove observers.
   virtual void AddObserver(Observer* observer) {}
   virtual void RemoveObserver(Observer* observer) {}
   virtual bool HasObservers() const;
 
   // Whether the store has completed all asynchronous initialization.
   virtual bool IsInitializationComplete() const;
 
   // Get the value for a given preference |key| and stores it in |*result|.
   // |*result| is only modified if the return value is true and if |result|
   // is not NULL. Ownership of the |*result| value remains with the PrefStore.
   virtual bool GetValue(const std::string& key,
                         const base::Value** result) const = 0;
 
  protected:
   friend class base::RefCounted<PrefStore>;
   virtual ~PrefStore() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PrefStore);
 };
 
 #endif  // BASE_PREFS_PREF_STORE_H_