sync: move invalidation version prefs out of SyncPrefs into InvalidatorStorage.

sync/tools/sync_listen_notifications.cc

Index: sync/tools/sync_listen_notifications.cc
diff --git a/sync/tools/sync_listen_notifications.cc b/sync/tools/sync_listen_notifications.cc
index 0b8f3451a3d885475c058ca4107b588f9138ce92..d8521631e33e762619f4bae6c4145ecc18e6a7d1 100644
--- a/sync/tools/sync_listen_notifications.cc
+++ b/sync/tools/sync_listen_notifications.cc
@@ -84,6 +84,14 @@ class NullInvalidationStateTracker
               << syncable::ModelTypeToString(model_type) << " to "
               << max_invalidation_version;
   }
+
+  virtual std::string GetInvalidationState() const OVERRIDE {
+    return std::string();
+  }
+
+  virtual void SetInvalidationState(const std::string& state) OVERRIDE {
+    LOG(INFO) << "Setting invalidation state to: " << state;
+  }
 };
 
 const char kEmailSwitch[] = "email";