Fix lo_space initialization for external snapshot

R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/684263002

Patch from André Baixo <baixo@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24998 ce2b1a6d-e550-0410-aec6-3dcde31c8c00