DevTools: [Console] fix console context menu handler

A fix to exception which was risen on right-click over console message.
This is a bug caused by r172651.

NOTRY=true

Review URL: https://codereview.chromium.org/256913003

git-svn-id: svn://svn.chromium.org/blink/trunk@172702 bbb929c8-8fbe-4397-9dbb-9b2b20218538