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

mojo/edk/embedder/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.
 
 source_set("embedder") {
   # This isn't really a standalone target, it must be linked into the
   # mojo_system_impl component.
   visibility = [ "//mojo/edk/system" ]
 
-  deps = [ "//base", ]
+  deps = [
+    "//base",
+    "//mojo/public/cpp/system",
+  ]
 
   defines = [
     "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
     "MOJO_SYSTEM_IMPLEMENTATION",
   ]
 
   configs += [ "//mojo/edk/system:system_config" ]
 
   sources = [
     "channel_info_forward.h",
@@ skipping 42 matching lines @@
     "//mojo/edk/system",
     "//testing/gtest",
   ]
 
   sources = [
     "embedder_unittest.cc",
     "platform_channel_pair_posix_unittest.cc",
     "simple_platform_shared_buffer_unittest.cc",
   ]
 }