Revert of Deletes mojo::Callback

mojo/mojo_public.gypi

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'mojo_cpp_bindings_sources': [
       'public/cpp/bindings/array.h',
       'public/cpp/bindings/array_traits.h',
       'public/cpp/bindings/array_traits_carray.h',
       'public/cpp/bindings/array_traits_standard.h',
       'public/cpp/bindings/array_traits_stl.h',
       'public/cpp/bindings/associated_binding.h',
       'public/cpp/bindings/associated_group.h',
       'public/cpp/bindings/associated_interface_ptr.h',
       'public/cpp/bindings/associated_interface_ptr_info.h',
       'public/cpp/bindings/associated_interface_request.h',
       'public/cpp/bindings/binding.h',
       'public/cpp/bindings/binding_set.h',
+      'public/cpp/bindings/callback.h',
       'public/cpp/bindings/enum_traits.h',
       'public/cpp/bindings/interface_ptr.h',
       'public/cpp/bindings/interface_ptr_set.h',
       'public/cpp/bindings/interface_request.h',
       'public/cpp/bindings/lib/array_internal.cc',
       'public/cpp/bindings/lib/array_internal.h',
       'public/cpp/bindings/lib/array_serialization.h',
       'public/cpp/bindings/lib/associated_group.cc',
       'public/cpp/bindings/lib/associated_interface_ptr_state.h',
       'public/cpp/bindings/lib/binding_state.h',
@@ skipping 108 matching lines @@
       'public/c/system/message_pipe.h',
       'public/c/system/platform_handle.h',
       'public/c/system/system_export.h',
       'public/c/system/thunks.cc',
       'public/c/system/thunks.h',
       'public/c/system/types.h',
       'public/c/system/wait_set.h',
     ],
   },
 }