[Sync] Replace typedef with using.

components/sync/test/fake_server/sessions_hierarchy.h

Index: components/sync/test/fake_server/sessions_hierarchy.h
diff --git a/components/sync/test/fake_server/sessions_hierarchy.h b/components/sync/test/fake_server/sessions_hierarchy.h
index 2163fb5f8be27e8267494fc7b3cec0e13ba28197..8d323d499b6fc48f684207e8d83bda2607b6fffe 100644
--- a/components/sync/test/fake_server/sessions_hierarchy.h
+++ b/components/sync/test/fake_server/sessions_hierarchy.h
@@ -47,11 +47,11 @@ class SessionsHierarchy {
 
  private:
   // A collection of tab URLs.
-  typedef std::multiset<std::string> Window;
+  using Window = std::multiset<std::string>;
 
   // A collection of Windows (an instance of this collection represents a
   // sessions hierarchy).
-  typedef std::multiset<Window> WindowContainer;
+  using WindowContainer = std::multiset<Window>;
 
   // The windows of the sessions hierarchy.
   WindowContainer windows_;