Add BUILD.gn for third_party/yasm.

build/gn_run_binary.py

Index: build/gn_run_binary.py
diff --git a/build/gn_run_binary.py b/build/gn_run_binary.py
new file mode 100644
index 0000000000000000000000000000000000000000..39f871bf58a95c1c131e27a1094bfef90fad362a
--- /dev/null
+++ b/build/gn_run_binary.py
@@ -0,0 +1,13 @@
+# Copyright 2014 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.
+
+"""Helper script for GN to run an arbitrary binary."""
+
+import sys
+import subprocess
+
+args = [ './' + sys.argv[1] ]
+args.extend(sys.argv[2:])
+
+subprocess.call(args)