In GrowMode, force the value to the right representation to avoid deopts between storing the length and storing the value.

BUG=16459193
LOG=n
R=danno@chromium.org

Review URL: https://codereview.chromium.org/419683004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22616 ce2b1a6d-e550-0410-aec6-3dcde31c8c00