Use heap iterator in store buffer when page was swept precisely.

BUG=
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/413693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22592 ce2b1a6d-e550-0410-aec6-3dcde31c8c00