OffscreenCanvas should be listed in the CanvasImageSource typedef in CanvasDrawImage.idl and CanvasFillStrokeStyles.idl. Implementation-wise, all of the code is already there, but it has to be broadened from covering only HTMLCanvasElement objects to covering CanvasBase instead.
Created attachment 334142 [details] WIP patch
Created attachment 380625 [details] Patch
Created attachment 384301 [details] Patch
Comment on attachment 384301 [details] Patch r=me
Comment on attachment 384301 [details] Patch Clearing flags on attachment: 384301 Committed r252864: <https://trac.webkit.org/changeset/252864>
All reviewed patches have been landed. Closing bug.
<rdar://problem/57475517>