Teach PRESUBMIT.py how to run gn check for most of the mojo repo

mojo/spy/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("//mojo/public/tools/bindings/mojom.gni")
 
 source_set("spy") {
   deps = [
     ":spy_bindings",
     "//base",
+    "//mojo/application_manager",
+    "//mojo/public/cpp/bindings",
+    "//mojo/public/cpp/system",
+    "//net",
     "//net:http_server",
     "//url",
-    "//mojo/application_manager",
   ]
 
   sources = [
     "common.h",
     "spy.cc",
     "spy.h",
     "spy_server_impl.h",
     "spy_server_impl.cc",
     "websocket_server.cc",
     "websocket_server.h",
   ]
 }
 
 mojom("spy_bindings") {
   sources = [
     "public/spy.mojom",
   ]
 }