Revert of Deletes mojo::Callback

components/mus/ws/window_tree.cc

Index: components/mus/ws/window_tree.cc
diff --git a/components/mus/ws/window_tree.cc b/components/mus/ws/window_tree.cc
index 285faec418a14b724d4aff4fc8b3f71bdcf9ae4b..fa53c986fb4948d03becb6879f2dc28db6a82c7a 100644
--- a/components/mus/ws/window_tree.cc
+++ b/components/mus/ws/window_tree.cc
@@ -31,6 +31,7 @@
 #include "ui/platform_window/text_input_state.h"
 
 using mojo::Array;
+using mojo::Callback;
 using mojo::InterfaceRequest;
 using mojo::String;
 
@@ -1119,7 +1120,7 @@
 
 void WindowTree::GetWindowTree(
     Id window_id,
-    const base::Callback<void(Array<mojom::WindowDataPtr>)>& callback) {
+    const Callback<void(Array<mojom::WindowDataPtr>)>& callback) {
   std::vector<const ServerWindow*> windows(
       GetWindowTree(ClientWindowId(window_id)));
   callback.Run(WindowsToWindowDatas(windows));