Make memento checks more stable. Add filler at the end of new space and check if object and memento are on the same new space page.

BUG=
R=bmeurer@chromium.org, mvstanton@chromium.org

Review URL: https://codereview.chromium.org/152613002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19024 ce2b1a6d-e550-0410-aec6-3dcde31c8c00