Keep CodeRange::current_allocation_block_index_ in range after r21869.

BUG=305878,388328
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/354003003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22037 ce2b1a6d-e550-0410-aec6-3dcde31c8c00