WIP: Pref service: use the incognito support from Chrome

components/prefs/pref_value_store.h

Index: components/prefs/pref_value_store.h
diff --git a/components/prefs/pref_value_store.h b/components/prefs/pref_value_store.h
index 79767d3ad09af50d1f0f4262e039b35fe6e2ae0e..ee407ca2e04b1046c7ee3193de0fdef2e64c354e 100644
--- a/components/prefs/pref_value_store.h
+++ b/components/prefs/pref_value_store.h
@@ -80,7 +80,9 @@ class COMPONENTS_PREFS_EXPORT PrefValueStore {
                  PrefStore* user_prefs,
                  PrefStore* recommended_prefs,
                  PrefStore* default_prefs,
-                 PrefNotifier* pref_notifier);
+                 PrefNotifier* pref_notifier,
+                 PrefStore* incognito_prefs = nullptr);
+  // TODO(tibell): Add incognito store as an option.
   virtual ~PrefValueStore();
 
   // Creates a clone of this PrefValueStore with PrefStores overwritten
@@ -92,7 +94,8 @@ class COMPONENTS_PREFS_EXPORT PrefValueStore {
                                      PrefStore* user_prefs,
                                      PrefStore* recommended_prefs,
                                      PrefStore* default_prefs,
-                                     PrefNotifier* pref_notifier);
+                                     PrefNotifier* pref_notifier,
+                                     PrefStore* incognito_prefs = nullptr);
 
   // A PrefValueStore can have exactly one callback that is directly
   // notified of preferences changing in the store. This does not