Original review: http://codereview.chromium.org/646010/show

Change the implementation of lastIndexOf method of String. Convert the
strings in question to Vector<char> or Vector<uc16> and execute simple
search algorithm on them.

The difference in speed on 500k character string is about 10x.

TBR=ager@chromium.org

Review URL: http://codereview.chromium.org/650036

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3922 ce2b1a6d-e550-0410-aec6-3dcde31c8c00