This interface remains only for the constants, for a context, use the
WebIDL/Paris bindings instead (CanvasRenderingContext2D.webidl).
The constants are used by CanvasRenderingContext2D::DrawWindow and are
used in WindowsPreviewPerTab.jsm and some extensions. The constants can
be referenced directly via a canvas context 2d rather than this interface,
and that should be preferred in new code.