Improve GN header checker, make //ui pass.

ui/views_content_client/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("//build/config/ui.gni")
 
 component("views_content_client") {
   testonly = true
   sources = [
     "views_content_browser_client.cc",
@@ skipping 17 matching lines @@
     "//content/shell:content_shell_lib",
     "//third_party/icu",
     "//ui/base",
     "//ui/events",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/resources",
     "//ui/resources:ui_test_pak",
     "//ui/views",
     "//ui/views:test_support",
+    "//ui/wm",
   ]
 
   if (use_aura) {
     sources += [
       "views_content_client_main_parts_aura.cc",
       "views_content_client_main_parts_aura.h",
     ]
 
     if (!is_chromeos) {
       sources += [ "views_content_client_main_parts_desktop_aura.cc" ]
     }
 
     deps += [ "//ui/aura" ]
   }
 }