Do not avoid flattening cons string when creating a string slice.

R=ulan@chromium.org
BUG=364656
LOG=N

Review URL: https://codereview.chromium.org/247093002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20882 ce2b1a6d-e550-0410-aec6-3dcde31c8c00