Format GN files and add a presubmit check that GN is properly formatted

build/config/BUILD.gn

 # Copyright (c) 2013 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.
 
 # Debug/release ----------------------------------------------------------------
 
 config("debug") {
-  defines = [
-    "DEBUG",
-  ]
+  defines = [ "DEBUG" ]
 }
 
 config("release") {
-  defines = [
-    "NDEBUG"
-  ]
+  defines = [ "NDEBUG" ]
 }
 
 config("product") {
   defines = [
     "NDEBUG",
     "PRODUCT",
   ]
 }