Fix a race condition in preference metric reporting.

chrome/browser/prefs/synced_pref_change_registrar.h

 // Copyright 2013 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 CHROME_BROWSER_PREFS_SYNCED_PREF_CHANGE_REGISTRAR_H_
 #define CHROME_BROWSER_PREFS_SYNCED_PREF_CHANGE_REGISTRAR_H_
 
 #include <map>
 #include <string>
 
 #include "base/callback.h"
-#include "chrome/browser/prefs/pref_service_syncable.h"
 #include "chrome/browser/prefs/synced_pref_observer.h"
 
+class PrefServiceSyncable;
+
 // Manages the registration of one or more SyncedPrefObservers on a
 // PrefServiceSyncable. This is modeled after base::PrefChangeRegistrar, and
 // it should be used whenever it's necessary to determine whether a pref change
 // has come from sync or from some other mechanism (managed, UI, external, etc.)
 class SyncedPrefChangeRegistrar : public SyncedPrefObserver {
  public:
   // Registered callbacks may optionally take a path argument.
   // The boolean argument indicates whether (true) or not (false)
   // the change was a result of syncing.
   typedef base::Callback<void(bool)> ChangeCallback;
@@ skipping 23 matching lines @@
 
   typedef std::map<std::string, NamedChangeCallback> ObserverMap;
 
   PrefServiceSyncable* pref_service_;
   ObserverMap observers_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncedPrefChangeRegistrar);
 };
 
 #endif  // CHROME_BROWSER_PREFS_SYNCED_PREF_CHANGE_REGISTRAR_H_