DevTools: Add missing jsdocs for Target members and fix revealed errors.

This reveals and fixes a REGRESSION introduced in Blink r170439.

R=pfeldman@chromium.org, pfeldman, sergeyv@chromium.org

Review URL: https://codereview.chromium.org/247333004

git-svn-id: svn://svn.chromium.org/blink/trunk@172349 bbb929c8-8fbe-4397-9dbb-9b2b20218538