-[WKWebViewConfiguration copyWithZone] doesn't copy the _groupIdentifier string, which means WKWebView's created with a copy of the configuration won't have a page group. For example, this is done during UIDelegate::UIClient::createNewPage().
<rdar://problem/35891437>
Created attachment 328633 [details] Patch
Comment on attachment 328633 [details] Patch r=me
Fixed in <https://trac.webkit.org/changeset/225603/webkit>.