Heap-use-after-free in WebCore::Resource::checkNotify

Backed out of some overzealous shape-outside related postDestroy() changes in RenderObject.cpp (which I'd already backed out of in the WebKit version of the same code). This is a speculative fix.

BUG=362898

Review URL: https://codereview.chromium.org/232773014

git-svn-id: svn://svn.chromium.org/blink/trunk@171712 bbb929c8-8fbe-4397-9dbb-9b2b20218538