Initial implementation of WebGLSync

This patch adds initial support for WebGLSync.

Note:
There is no test for the isSync, deleteSync and waitSync functions in the `conformance2/sync/sync-webgl-specific.html`.

components/script/dom/webidls/WebGLSync.webidl

@@ -0,0 +1,11 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+//
+// WebGL IDL definitions scraped from the Khronos specification:
+// https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.14
+//
+
+[Pref="dom.webgl2.enabled"]
+interface WebGLSync : WebGLObject {
+};