Add BUILD.gn file to build libpng.

tools/gn/secondary/third_party/libpng/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.
+
+config("libpng_config") {
+  include_dirs = [
+    ".",
+  ]
+
+  defines = [
+    "CHROME_PNG_WRITE_SUPPORT",
+    "PNG_USER_CONFIG",
+  ]
+  if (is_android) {
+    #'toolsets': ['target', 'host'],
+    defines += [
+      "CHROME_PNG_READ_PACK_SUPPORT",  # Required by freetype.
+    ]
+  }
+}
+
+static_library("libpng") {
+  sources =  [
+    "png.c",
+    "png.h",
+    "pngconf.h",
+    "pngerror.c",
+    "pnggccrd.c",
+    "pngget.c",
+    "pngmem.c",
+    "pngpread.c",
+    "pngread.c",
+    "pngrio.c",
+    "pngrtran.c",
+    "pngrutil.c",
+    "pngset.c",
+    "pngtrans.c",
+    "pngusr.h",
+    "pngvcrd.c",
+    "pngwio.c",
+    "pngwrite.c",
+    "pngwtran.c",
+    "pngwutil.c",
+  ]
+
+  defines = [
+    "CHROME_PNG_WRITE_SUPPORT",
+    "PNG_USER_CONFIG",
+  ]
+
+  direct_dependent_configs = [ ":libpng_config" ]
+
+  configs -= "//build/config/compiler:chromium_code"
+  configs += "//build/config/compiler:no_chromium_code"
+
+  #'export_dependent_settings': [

[brettw, 2014/01/08 05:26:50]

This would turn into:
forward_dependent_configs_from = [ "//third_party/zlib" ]

[tfarina, 2014/01/09 01:18:42]

Done.

+  #  '../zlib/zlib.gyp:zlib',
+  #],
+
+  if (is_win) {
+    # TODO(jschuh): http://crbug.com/167187
+    #'msvs_disabled_warnings': [ 4267 ],
+    #'type': '<(component)',
+
+    #if (component=="shared_library"') {

[brettw, 2014/01/08 05:26:50]

Substitute component -> component_mode and I think this will work.

[tfarina, 2014/01/09 01:18:42]

Done.

+#       defines = [
+#        "PNG_BUILD_DLL",
+#        "PNG_NO_MODULEDEF",
+#       ]
+#       'direct_dependent_settings': {

[brettw, 2014/01/08 05:26:50]

Put this in the config above (in a component mode condition).

[tfarina, 2014/01/09 01:18:42]

Done.

+#         defines = [
+#             "PNG_USE_DLL",
+#          ]
+#       },
+  } else {
+    #['OS!="win"', {'product_name': 'png'}],

[brettw, 2014/01/08 05:26:50]

I'd just delete this.

[tfarina, 2014/01/09 01:18:42]

Done.

+    #'type': 'static_library',
+  }
+
+  deps =  [
+    "//third_party/zlib",
+  ]
+}