unbundle: add GN files for ffmpeg, libevent, libxslt, snappy, and yasm

build/linux/unbundle/ffmpeg.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",
+  ]
+}
+
+shim_headers("ffmpeg_shim") {
+  root_path = "."
+  headers = [
+    "libavcodec/avcodec.h",
+    "libavformat/avformat.h",
+    "libavutil/imgutils.h",
+  ]
+}
+
+source_set("ffmpeg") {
+  deps = [
+    ":ffmpeg_shim",
+  ]
+  public_configs = [ ":system_ffmpeg" ]
+}