Add gclient hook to download prebuilt mojo_shell

mojo/shell/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("//build/config/ui.gni")
+import("//mojo/public/mojo.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
-import("//build/config/ui.gni")
 
+if (!use_prebuilt_mojo_shell) {
+
+# GYP version: mojo/mojo.gyp:mojo_shell
 executable("mojo_shell") {
   deps = [
     ":lib",
     "//base",
     "//mojo/common",
     "//mojo/environment:chromium",
   ]
 
   if (is_component_build) {
     deps += ["//ui/gl"]
   }
 
   sources = [
     "desktop/mojo_main.cc"
   ]
 }
 
+}  # !use_prebuilt_mojo_shell
+
 # GYP version: mojo/mojo.gyp:mojo_shell_lib
 source_set("lib") {
   deps = [
     ":app_child_process_bindings",
     ":external_application_registrar_bindings",
     ":external_service_bindings",
     "//base",
     "//base/third_party/dynamic_annotations",
     "//base:base_static",
     "//mojo/application",
@@ skipping 156 matching lines @@
     "//mojo/edk/system",
     "//mojo/environment:chromium",
   ]
 
   sources = [
     "incoming_connection_listener_unittest.cc",
     "external_application_listener_unittest.cc",
     "external_application_test_main.cc",
   ]
 }