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

build/linux/unbundle/libpng.gn

Index: build/linux/unbundle/libpng.gn
diff --git a/build/linux/unbundle/ffmpeg.gn b/build/linux/unbundle/libpng.gn
similarity index 50%
copy from build/linux/unbundle/ffmpeg.gn
copy to build/linux/unbundle/libpng.gn
index 9a4fbfd75a40957c3430f0d8c9e8d006892372c1..9ae5abe076ca1efa46c6ce3b4271de0b8dc4f2fd 100644
--- a/build/linux/unbundle/ffmpeg.gn
+++ b/build/linux/unbundle/libpng.gn
@@ -5,26 +5,21 @@
 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("ffmpeg_shim") {
+shim_headers("libpng_shim") {
   root_path = "."
   headers = [
-    "libavcodec/avcodec.h",
-    "libavformat/avformat.h",
-    "libavutil/imgutils.h",
+    "png.h",
+    "pngconf.h",
   ]
 }
 
-source_set("ffmpeg") {
+source_set("libpng") {
   deps = [
-    ":ffmpeg_shim",
+    ":libpng_shim",
   ]
-  public_configs = [ ":system_ffmpeg" ]
+  public_configs = [ ":system_libpng" ]
 }