ImageBuffer::backendSize() waits for GPUP to create the backend
<rdar://problem/116117019>
Pull request: https://github.com/WebKit/WebKit/pull/18275
Committed 268631@main (f77323c9d2ad): <https://commits.webkit.org/268631@main> Reviewed commits have been landed. Closing PR #18275 and removing active labels.