The trouble is that the AsHandle method can allocate and thus
invoke GC, which could empty the table while we are updating it.

BUG=chromium:430880
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/700913003

Cr-Commit-Position: refs/heads/master@{#25229}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25229 ce2b1a6d-e550-0410-aec6-3dcde31c8c00