Introduce ServoParser

This is a common inline parent to ServoHTMLParser and ServoXMLParser.

components/script/dom/webidls/ServoParser.webidl

@@ -0,0 +1,10 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+[Exposed=(Window,Worker),
+ Inline]
+interface ServoParser {};