WebKit: use-after-free in WebCore::TreeScope::documentScope(CVE-2017-13796)

There is a use-after-free security vulnerability in WebKit. The vulnerability was confirmed on ASan build of WebKit nightly. ASan log: ================================================================= ==29647==ERROR: AddressSanitizer: heap-use-after-free on address 0x61e00005d0d8 at pc...