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_deduplicate.py

@@ -0,0 +1,20 @@
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+        interface Foo;
+        interface Bar;
+        interface Foo;
+        """
+    )
+
+    results = parser.finish()
+
+    # There should be no duplicate interfaces in the result.
+    expectedNames = sorted(["Foo", "Bar"])
+    actualNames = sorted(map(lambda iface: iface.identifier.name, results))
+    harness.check(
+        actualNames, expectedNames, "Parser shouldn't output duplicate names."
+    )