Add allocation sites to scratchpad only if a significant amount of mementos was found.

BUG=
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/284223007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21370 ce2b1a6d-e550-0410-aec6-3dcde31c8c00