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

build/linux/unbundle/re2.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/shim_headers.gni")
 
-shim_headers("snappy_shim") {
+shim_headers("re2_shim") {
   root_path = "src"
   headers = [
-    "snappy-c.h",
-    "snappy-sinksource.h",
-    "snappy-stubs-public.h",
-    "snappy.h",
+    "re2/filtered_re2.h",
+    "re2/re2.h",
+    "re2/set.h",
+    "re2/stringpiece.h",
+    "re2/variadic_function.h",
   ]
 }
 
-source_set("snappy") {
+source_set("re2") {
   deps = [
-    ":snappy_shim",
+    ":re2_shim",
   ]
-  libs = [ "snappy" ]
+  libs = [ "re2" ]
 }