Fix processing of partially initialized JSWeakCollection.

R=hpayer@chromium.org
BUG=v8:2070
LOG=N

Review URL: https://codereview.chromium.org/300843009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21563 ce2b1a6d-e550-0410-aec6-3dcde31c8c00