Port r10012 (09c6bcba81).

Original commit message:
There is no need to bail out to runtime system when creating sliced strings of external strings since the string content is unimportant.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8585041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10027 ce2b1a6d-e550-0410-aec6-3dcde31c8c00