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_identifier_conflicts.py

@@ -0,0 +1,16 @@
+def WebIDLTest(parser, harness):
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface ArgumentIdentifierConflict {
+              undefined foo(boolean arg1, boolean arg1);
+            };
+        """
+        )
+
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(threw, "Should have thrown.")