DevTools: Deleting object property in console would just assign it to undefined.

Evaluate explicit "delete this[p]" to delete a property.

R=vsevik

Review URL: https://codereview.chromium.org/323423002

git-svn-id: svn://svn.chromium.org/blink/trunk@176188 bbb929c8-8fbe-4397-9dbb-9b2b20218538