add missing externs to repo

externs/webextension_polyfill/webrequest/UrlClassification.hx

@@ -0,0 +1,12 @@
+package webextension_polyfill.webrequest;
+
+typedef UrlClassification = {
+	/**
+		Classification flags if the request has been classified and it is first party.
+	**/
+	var firstParty : UrlClassificationParty;
+	/**
+		Classification flags if the request has been classified and it or its window hierarchy is third party.
+	**/
+	var thirdParty : UrlClassificationParty;
+};