Add yasm and ffmpeg into the build tree for linux.

third_party/ffmpeg/dummy_nolink.cc

Index: third_party/ffmpeg/dummy_nolink.cc
diff --git a/third_party/ffmpeg/dummy_nolink.cc b/third_party/ffmpeg/dummy_nolink.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8659983478a4f1e2ce38fdd1d5b7aeb30b300081
--- /dev/null
+++ b/third_party/ffmpeg/dummy_nolink.cc
@@ -0,0 +1,10 @@
+// Copyright (c) 2009 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.
+
+// Dummy file to make the build system happy for the libffmpegsumo_nolink
+// target.
+
+int main(void) {
+  return 0;
+}