Use a union typdef for the return type of canvas.getContext

third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.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/#the-canvas-element
+
+typedef (CanvasRenderingContext2D or
+         WebGLRenderingContext or
+         WebGL2RenderingContext or
+         ImageBitmapRenderingContext) RenderingContext;
+
+partial interface HTMLCanvasElement
+{
+    // The PermissiveDictionaryConversion extended attribute is needed to allow the
+    // autogenerated code to match the behavior of the custom binding. Web IDL
+    // requires throwing TypeError if the incoming argument is not an object type
+    // (and is not undefined or null). The binding must ignore this.
+    // Related spec issue: https://github.com/whatwg/html/issues/595
+    RenderingContext? getContext(DOMString contextId, [PermissiveDictionaryConversion] optional CanvasContextCreationAttributes attributes);
+};