* Faster hashing for sequential strings.

 * When adding short external two-byte strings try to convert them
   back to ascii. Chances are high the embedder uses two-byte
   representation even for ascii strings. This optimization saves
   memory and makes hashing faster.

Review URL: http://codereview.chromium.org/1444001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00