Otherwise page header check is not quite robust: if there is a smi
at the same offset as Page::is_normal_page field, wrong result would
be returned.

That shouldn't be the problem for paged spaces as objects in those
pages do not span page boundaries and thus cannot mess with ::is_normal_page field.

Review URL: http://codereview.chromium.org/1175001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4270 ce2b1a6d-e550-0410-aec6-3dcde31c8c00