[WebGPU] Give a ConvertToBackingContext to SurfaceImpl and SwapChainImpl
<rdar://problem/104522936>
Pull request: https://github.com/WebKit/WebKit/pull/8942
Committed 259235@main (db1acf95baa0): <https://commits.webkit.org/259235@main> Reviewed commits have been landed. Closing PR #8942 and removing active labels.