Fix a race condition in preference metric reporting.

base/prefs/pref_store.h

Index: base/prefs/pref_store.h
diff --git a/base/prefs/pref_store.h b/base/prefs/pref_store.h
index b736ac3d79963533e37476812f9d59b6b40d3235..77044b48fac5c52ba17eaafd49d05fa0052a9c72 100644
--- a/base/prefs/pref_store.h
+++ b/base/prefs/pref_store.h
@@ -27,13 +27,12 @@ class BASE_PREFS_EXPORT PrefStore : public base::RefCounted<PrefStore> {
   // 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() {}