Execute a memory barrier when adding a new page to a space to synchronize access with concurrent sweepers.

BUG=

Review URL: https://codereview.chromium.org/12342017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00