FLAG_max_new_space_size is in MB.

Plus, cleanup of the space - generation mess. More to do there...

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/268073004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21148 ce2b1a6d-e550-0410-aec6-3dcde31c8c00