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_incomplete_parent.py

@@ -0,0 +1,21 @@
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+        interface TestIncompleteParent : NotYetDefined {
+          undefined foo();
+        };
+
+        interface NotYetDefined : EvenHigherOnTheChain {
+        };
+
+        interface EvenHigherOnTheChain {
+        };
+    """
+    )
+
+    parser.finish()
+
+    harness.ok(True, "TestIncompleteParent interface parsed without error.")