Simplify WebGLRenderingContextBase#canvas attribute

[saschanaz]

Web IDL should automatically drop the HTMLCanvasElement part from workers, as in ImageBitmapRenderingContext:

[Exposed=(Window,Worker)]
interface ImageBitmapRenderingContext {
  readonly attribute (HTMLCanvasElement or OffscreenCanvas) canvas;
  undefined transferFromImageBitmap(ImageBitmap? bitmap);
};

Thus there is no need to process it manually. (Doing so makes things hard for IDL processing tools actually, e.g. microsoft/TypeScript-DOM-lib-generator#1050.)

[kenrussell]

Thanks, looks good to me. Spec editor @jdashg should review too.