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

build/linux/unbundle/libevent.gn

Index: build/linux/unbundle/libevent.gn
diff --git a/mash/shelf/public/interfaces/BUILD.gn b/build/linux/unbundle/libevent.gn
similarity index 50%
copy from mash/shelf/public/interfaces/BUILD.gn
copy to build/linux/unbundle/libevent.gn
index 9d87ca48f54095da18feaf407768b1bda825ff45..47e48e9b9927cd25045b67faae928961863a0a18 100644
--- a/mash/shelf/public/interfaces/BUILD.gn
+++ b/build/linux/unbundle/libevent.gn
@@ -2,15 +2,16 @@
 # 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 = [
-    "//skia/public/interfaces",
+    ":libevent_shim",
   ]
+  libs = [ "event" ]
 }