mojo: Add promises for python bindings.

mojo/public/python/BUILD.gn

 # Copyright 2014 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.
 
 import("//third_party/cython/rules.gni")
 
 group("python") {
   deps = [
     ":base",
     ":bindings",
@@ skipping 32 matching lines @@
     "$root_out_dir/python/mojo/{{source_file_part}}",
   ]
 }
 
 # GYP version: mojo.gyp:mojo_python_bindings
 copy("bindings") {
   sources = [
     "mojo/bindings/__init__.py",
     "mojo/bindings/descriptor.py",
     "mojo/bindings/messaging.py",
+    "mojo/bindings/promise.py",
     "mojo/bindings/reflection.py",
     "mojo/bindings/serialization.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/bindings/{{source_file_part}}",
   ]
   deps = [
     ":base",
     ":system",
   ]
 }