unbundle: add GN files for libpng, libvpx, and re2

build/linux/unbundle/libpng.gn

 # Copyright 2016 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/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
-pkg_config("system_ffmpeg") {
-  packages = [
-    "libavcodec",
-    "libavformat",
-    "libavutil",
+pkg_config("system_libpng") {
+  packages = [ "libpng" ]
+}
+
+shim_headers("libpng_shim") {
+  root_path = "."
+  headers = [
+    "png.h",
+    "pngconf.h",
   ]
 }
 
-shim_headers("ffmpeg_shim") {
-  root_path = "."
-  headers = [
-    "libavcodec/avcodec.h",
-    "libavformat/avformat.h",
-    "libavutil/imgutils.h",
+source_set("libpng") {
+  deps = [
+    ":libpng_shim",
   ]
+  public_configs = [ ":system_libpng" ]
 }
-
-source_set("ffmpeg") {
-  deps = [
-    ":ffmpeg_shim",
-  ]
-  public_configs = [ ":system_ffmpeg" ]
-}