DevTools: fix crash on mac

This patch fixes crash in a rare scenario of pseudo element being
detached from DOM.

BUG=361274

Review URL: https://codereview.chromium.org/230483002

git-svn-id: svn://svn.chromium.org/blink/trunk@171137 bbb929c8-8fbe-4397-9dbb-9b2b20218538