Revert 23058 - "Allow externalizing cons and sliced string."

Reason for revert: broke inspector

| Fulfill overdue TODO.
|
| R=mstarzinger@chromium.org
|
| Review URL: https://codereview.chromium.org/460643002

TBR=yangguo@chromium.org,mstarzinger@chromium.org
BUG=403521
LOG=n

Review URL: https://codereview.chromium.org/470913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23115 ce2b1a6d-e550-0410-aec6-3dcde31c8c00