Fix crash due to unexpected Node deletion during MutationObserver registration book-keeping

R=inferno
BUG=260165

Review URL: https://chromiumcodereview.appspot.com/19303002

git-svn-id: svn://svn.chromium.org/blink/trunk@154264 bbb929c8-8fbe-4397-9dbb-9b2b20218538