Version 3.25.28.15 (merged r20882)

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

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

Review URL: https://codereview.chromium.org/258823011

git-svn-id: https://v8.googlecode.com/svn/branches/3.25@20999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00