Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

third_party/WebIDL/tests/test_variadic_callback.py

@@ -0,0 +1,13 @@
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+        callback TestVariadicCallback = any(any... arguments);
+    """
+    )
+
+    results = parser.finish()
+
+    harness.ok(True, "TestVariadicCallback callback parsed without error.")