Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/browser/prefs/pref_service.h

Index: chrome/browser/prefs/pref_service.h
diff --git a/chrome/browser/prefs/pref_service.h b/chrome/browser/prefs/pref_service.h
index 0d11873458a5d51c8cf80d3448846c31792941e4..9ea3787751571ab5275358c392088c09fd483aa1 100644
--- a/chrome/browser/prefs/pref_service.h
+++ b/chrome/browser/prefs/pref_service.h
@@ -183,7 +183,8 @@ class PrefService : public PrefServiceBase, public base::NonThreadSafe {
   void AddRegisteredPreference(const char* path,
                                base::Value* default_value);
 
-  void RemoveRegisteredPreference(const char* path);
+  // Updates local caches for a preference unregistered at |path|.
+  virtual void RemoveRegisteredPreference(const char* path);
 
   // The PrefNotifier handles registering and notifying preference observers.
   // It is created and owned by this PrefService. Subclasses may access it for
@@ -255,9 +256,4 @@ class PrefService : public PrefServiceBase, public base::NonThreadSafe {
   DISALLOW_COPY_AND_ASSIGN(PrefService);
 };
 
-// TODO(joi): Remove these forwards. They were placed here temporarily
-// to limit the size of the initial change that split
-// PrefServiceSimple and PrefServiceSyncable out of PrefService.
-#include "chrome/browser/prefs/pref_service_syncable.h"
-
 #endif  // CHROME_BROWSER_PREFS_PREF_SERVICE_H_