DevTools: bring back defense from self-xss

TBR=pfeldman

This patch restores self-xss protection functionality in console that
was regressed by
https://codereview.chromium.org/227433016

Review URL: https://codereview.chromium.org/239113005

git-svn-id: svn://svn.chromium.org/blink/trunk@171760 bbb929c8-8fbe-4397-9dbb-9b2b20218538