Fix for v8:3255 Grow KeyedStoreIC doesn't respect String value wrappers

BUG=v8:3255
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/226053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20527 ce2b1a6d-e550-0410-aec6-3dcde31c8c00