Remove internalized cons string types.

Currently, internalizing a cons string could result in either an
in-place converted internalized cons string or a newly created
internalized sequential string, depending on allocation success.

The former could end up being embedded into an IC, which is not
supported.

R=mstarzinger@chromium.org
BUG=357103
LOG=N

Review URL: https://codereview.chromium.org/218993011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20394 ce2b1a6d-e550-0410-aec6-3dcde31c8c00