Don't notify existing observers in SupervisedUserWhitelistService::AddSiteListsChangedCallback().

chrome/browser/supervised_user/supervised_user_url_filter.h

Index: chrome/browser/supervised_user/supervised_user_url_filter.h
diff --git a/chrome/browser/supervised_user/supervised_user_url_filter.h b/chrome/browser/supervised_user/supervised_user_url_filter.h
index a9f22b2fed1136023d0780afa5e668faaec1828e..3637c171b37ce954a4eb9f526764bd25fefff823 100644
--- a/chrome/browser/supervised_user/supervised_user_url_filter.h
+++ b/chrome/browser/supervised_user/supervised_user_url_filter.h
@@ -159,6 +159,10 @@ class SupervisedUserURLFilter
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
+  // Sets a different task runner for testing.
+  void SetBlockingTaskRunnerForTesting(
+      const scoped_refptr<base::TaskRunner>& task_runner);
+
  private:
   friend class base::RefCountedThreadSafe<SupervisedUserURLFilter>;
   ~SupervisedUserURLFilter();
@@ -191,6 +195,8 @@ class SupervisedUserURLFilter
 
   scoped_ptr<SupervisedUserAsyncURLChecker> async_url_checker_;
 
+  scoped_refptr<base::TaskRunner> blocking_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserURLFilter);
 };