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

build/linux/unbundle/libvpx.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_libvpx") {
+  packages = [ "vpx" ]
+}
+
+shim_headers("libvpx_shim") {
+  root_path = "source/libvpx"
+  headers = [
+    "vpx/vp8.h",
+    "vpx/vp8cx.h",
+    "vpx/vp8dx.h",
+    "vpx/vpx_codec.h",
+    "vpx/vpx_codec_impl_bottom.h",
+    "vpx/vpx_codec_impl_top.h",
+    "vpx/vpx_decoder.h",
+    "vpx/vpx_encoder.h",
+    "vpx/vpx_frame_buffer.h",
+    "vpx/vpx_image.h",
+    "vpx/vpx_integer.h",
+  ]
+}
+
+source_set("libvpx") {
+  deps = [
+    ":libvpx_shim",
+  ]
+  public_configs = [ ":system_libvpx" ]
+}