Add BUILD.gn file to build libpng.

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 13 matching lines @@
     "//components/translate:translate_common",
     "//crypto",
     "//device/usb:device_usb",
     "//ipc",
     "//net",
     "//net/third_party/nss:ssl",
     "//sdch",
     "//third_party/icu:icudata",
     "//third_party/leveldatabase",
     "//third_party/zlib",
+    "//third_party/libpng",
     "//third_party/WebKit/Source/weborigin",
     "//skia",
     "//tools/gn",
     "//url",
   ]
 }