DevTools: fix exception in mouse hovering codepath.

This patch fixes codepath that resolves object for popover to not
throw an exception in certain cases (see bug for details).

BUG=321383

Review URL: https://codereview.chromium.org/76003008

git-svn-id: svn://svn.chromium.org/blink/trunk@162323 bbb929c8-8fbe-4397-9dbb-9b2b20218538