webgl: Implement WebGLContextEvent and use it on context creation error

spec: https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15

components/script/dom/webidls/WebGLContextEvent.webidl

@@ -0,0 +1,15 @@
+/* -*- 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://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
+[Constructor(DOMString type, optional WebGLContextEventInit eventInit)]
+interface WebGLContextEvent : Event {
+    readonly attribute DOMString statusMessage;
+};
+
+// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
+dictionary WebGLContextEventInit : EventInit {
+    DOMString statusMessage;
+};