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

mojo/edk/system/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.
 
 config("system_config") {
   defines = [
     # Ensures that dependent projects import the core functions on Windows.
     "MOJO_USE_SYSTEM_IMPL",
   ]
 }
 
 component("system") {
   output_name = "mojo_system_impl"
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
-    "//mojo/edk/embedder",
+    "//mojo/public/c/system",
   ]
 
+  public_deps = [ "//mojo/edk/embedder" ]
+
   defines = [
     "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
     "MOJO_SYSTEM_IMPLEMENTATION",
   ]
 
   all_dependent_configs = [ ":system_config" ]
 
   sources = [
     "channel.cc",
     "channel.h",
@@ skipping 56 matching lines @@
     "waiter_list.cc",
     "waiter_list.h",
   ]
 }
 
 # GYP version: mojo/edk/mojo_edk.gyp:mojo_system_unittests
 test("mojo_system_unittests") {
   deps = [
     ":system",
     "//base",
+    "//base/test:test_support",
     "//mojo/edk/embedder:embedder_unittests",
     "//mojo/edk/test:test_support",
+    "//mojo/public/c/system",
     "//testing/gtest",
   ]
 
   sources = [
     "../test/multiprocess_test_helper_unittest.cc",
     "channel_endpoint_id_unittest.cc",
     "channel_unittest.cc",
     "core_test_base.cc",
     "core_test_base.h",
     "core_unittest.cc",
@@ skipping 34 matching lines @@
   ]
 
   sources = [
     "message_pipe_perftest.cc",
     "message_pipe_test_utils.h",
     "message_pipe_test_utils.cc",
     "test_utils.cc",
     "test_utils.h",
   ]
 }