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

build/linux/unbundle/libevent.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("//mojo/public/tools/bindings/mojom.gni")
+import("//build/shim_headers.gni")
 
-mojom("interfaces") {
-  sources = [
-    "shelf.mojom",
-    "shelf_constants.mojom",
+shim_headers("libevent_shim") {
+  root_path = "."
+  headers = [ "event.h" ]
+}
+
+source_set("libevent") {
+  deps = [
+    ":libevent_shim",
   ]
-
-  deps = [
-    "//skia/public/interfaces",
-  ]
+  libs = [ "event" ]
 }