Add the getContextAttributes interface from CanvasRenderingContext2D to OffscreenCanvasRenderingContext2D.
There is a lot of duplicated spec text for CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D, especially related to how CanvasRenderingContext2DSettings is handled. As with all things that are duplicated, there are accidental divergences. Among the most prominent accidental divergences is that getContextAttributes was accidentally not added to OffscreenCanvasRenderingContext2D when it was added to CanvasRenderingContext2D. This fixes that (and the spec fixed several other divergences that were also accidental, but Chrome had accidentally not diverged the implementation, e.g, willReadFrequently).