Add a PRESUBMIT for gn check, currently only for //sky/*

sky/services/inspector/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")
 
 group("inspector") {
   testonly = true
 
   deps = [
     ":sky_inspector_server",
   ]
 }
 
 shared_library("sky_inspector_server") {
   sources = [
     "inspector_frontend_impl.cc",
     "inspector_frontend_impl.h",
     "server.cc",
   ]
 
   deps = [
     "//base",
     "//mojo/application",
     "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
+    "//net",
     "//net:http_server",
     ":bindings",
   ]
 }
 
 mojom("bindings") {
   sources = [
     "inspector.mojom",
   ]
 }