WebKit: UXSS: CachedFrame doesn't detach openers(CVE-2017-2528)

When a document loads "about:blank" or "about:srcdoc", it tries to inherit the security origin from its parent frame, or its opener frame if the parent frame doesn't exist. Normally, it doesn't happen that a subframe's document inherits its opener frame's security origin, because it has the paren...