Combine %_SubString and %_StringCharAt.

R=ulan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12217071

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13700 ce2b1a6d-e550-0410-aec6-3dcde31c8c00