[WebGPU] Extra deref of SwapChainImpl::m_backing
<rdar://problem/104522855>
<rdar://problem/104522929>
Pull request: https://github.com/WebKit/WebKit/pull/8941
Committed 259318@main (83826f3fef40): <https://commits.webkit.org/259318@main> Reviewed commits have been landed. Closing PR #8941 and removing active labels.