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_argument_keywords.py

@@ -0,0 +1,22 @@
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+    interface Foo {
+      undefined foo(object constructor);
+    };
+    """
+    )
+
+    results = parser.finish()
+    harness.check(len(results), 1, "Should have an interface")
+    iface = results[0]
+    harness.check(len(iface.members), 1, "Should have an operation")
+    operation = iface.members[0]
+    harness.check(len(operation.signatures()), 1, "Should have one signature")
+    (retval, args) = operation.signatures()[0]
+    harness.check(len(args), 1, "Should have an argument")
+    harness.check(
+        args[0].identifier.name,
+        "constructor",
+        "Should have an identifier named 'constructor'",
+    )