Improve fast to slow elements conversion:

Improve fast to slow elements conversion:

o Use a more strict limit for old arrays.

o Initial capacity of a slow elements dictionary should be the number
  of used elements and not the old array capacity.

R=danno@chromium.org

Review URL: http://codereview.chromium.org/7464032

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8744 ce2b1a6d-e550-0410-aec6-3dcde31c8c00