Fix various flags in the Win64 GN build

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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 345 matching lines @@
       "//mojo",
       "//pdf",
       "//ppapi:ppapi_c",
       "//printing",
       "//remoting/client/plugin",
       "//sandbox",
       "//storage/browser",
       "//sync",
       "//third_party/WebKit/public:all_blink",
       "//third_party/angle:translator",
+      "//third_party/ffmpeg",
       "//third_party/leveldatabase",
       "//third_party/libjingle",
       "//third_party/usrsctp",
       "//third_party/webrtc",
       "//ui/accessibility",
       "//ui/app_list",
       "//ui/aura",
       "//ui/base",
       "//ui/display",
       "//ui/events",
       "//ui/gfx",
       "//ui/gl",
       "//ui/keyboard",
       "//ui/native_theme",
       "//ui/snapshot",
       "//ui/surface",
       "//ui/views",
       "//ui/views/controls/webview",
       "//ui/web_dialogs",
       "//ui/wm",
     ]
   }
 }