Do not update survival rate if the new space size is zero.

BUG=v8:2333
LOG=N
R=hpayer@chromium.org

Review URL: https://chromiumcodereview.appspot.com/48443002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17753 ce2b1a6d-e550-0410-aec6-3dcde31c8c00