Correct the OffscreenCanvas.getContext API

third_party/WebKit/Source/modules/offscreencanvas/OffscreenCanvasModules.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://html.spec.whatwg.org/multipage/scripting.html#the-offscreencanvas-interface
 
 typedef (OffscreenCanvasRenderingContext2D or
          WebGLRenderingContext or
          WebGL2RenderingContext) OffscreenRenderingContext;
+enum OffscreenRenderingContextType { "2d", "webgl", "webgl2" };
 
 partial interface OffscreenCanvas {
-    [CallWith=ScriptState, RaisesException, RuntimeEnabled=ExperimentalCanvasFeatures] OffscreenRenderingContext? getContext(DOMString contextId, optional CanvasContextCreationAttributes attributes);
+    [CallWith=ScriptState, RaisesException, RuntimeEnabled=ExperimentalCanvasFeatures] OffscreenRenderingContext? getContext(OffscreenRenderingContextType contextType, optional CanvasContextCreationAttributes attributes);
 };