Add a scoped notifier for pref dictionaries / lists.

chrome/browser/pref_service.h

Index: chrome/browser/pref_service.h
diff --git a/chrome/browser/pref_service.h b/chrome/browser/pref_service.h
index 85dd42ca8f7858ef2e18d44814ea8e9bc49b56bb..f1819adf4e6973f1cfbaaae9089903dac1cae158 100644
--- a/chrome/browser/pref_service.h
+++ b/chrome/browser/pref_service.h
@@ -27,6 +27,7 @@
 
 class NotificationObserver;
 class Preference;
+class ScopedPrefUpdate;
 
 class PrefService : public NonThreadSafe,
                     public ImportantFileWriter::DataSerializer {
@@ -158,7 +159,8 @@ class PrefService : public NonThreadSafe,
   // This method returns NULL only if you're requesting an unregistered pref or
   // a non-dict/non-list pref.
   // WARNING: Changes to the dictionary or list will not automatically notify
-  // pref observers. TODO(tc): come up with a way to still fire observers.
+  // pref observers.
+  // Use a ScopedPrefUpdate to update observers on changes.
   DictionaryValue* GetMutableDictionary(const wchar_t* path);
   ListValue* GetMutableList(const wchar_t* path);
 
@@ -216,6 +218,8 @@ class PrefService : public NonThreadSafe,
       PrefObserverMap;
   PrefObserverMap pref_observers_;
 
+  friend class ScopedPrefUpdate;
+
   DISALLOW_COPY_AND_ASSIGN(PrefService);
 };