heap: allow allocation in gc prologue/epilogue

BUG=
R=hpayer@chromium.org, danno@chromium.org

Review URL: https://codereview.chromium.org/177243012

Patch from Fedor Indutny <fedor.indutny@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19978 ce2b1a6d-e550-0410-aec6-3dcde31c8c00