Changing string length field  type from int to SMI. It will make it be a regular field. Code generated in EmitNamedLoad could be patched for faster access to string.length.

Review URL: http://codereview.chromium.org/1706013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4581 ce2b1a6d-e550-0410-aec6-3dcde31c8c00