Simplify line editor choice in d8.

R=rossberg@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12330171

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13782 ce2b1a6d-e550-0410-aec6-3dcde31c8c00