Implement TextEncoder

Fixes #4768.

components/script/dom/webidls/TextEncoder.webidl

@@ -0,0 +1,12 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+/* https://encoding.spec.whatwg.org/#interface-textencoder */
+[Constructor(optional DOMString utfLabel = "utf-8")/*, Exposed=Window,Worker */]
+interface TextEncoder {
+   readonly attribute DOMString encoding;
+   [NewObject]
+   Uint8Array encode(optional USVString input = "");
+};