when using snapshots.

The alignment of new space has to match the alignment in the snapshot,
but the max committed amount of memory does not.

For now, we assume that the default semispace size is always used in a
snapshot.
Review URL: http://codereview.chromium.org/300036

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3106 ce2b1a6d-e550-0410-aec6-3dcde31c8c00