Split collections flag into weak and non-weak.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/201593004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20019 ce2b1a6d-e550-0410-aec6-3dcde31c8c00