Fix crash in blink::DOMWindowEventQueue::close

BUG=395991
R=yurys, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/453943003

git-svn-id: svn://svn.chromium.org/blink/trunk@179929 bbb929c8-8fbe-4397-9dbb-9b2b20218538