Oilpan: Don't free collection backings that have been allocated on a different thread.

This fixes some WorkerClients related crashes that make use of safe cross-thread manipulation of collections.

BUG=408101
R=ager@chromium.org

Review URL: https://codereview.chromium.org/507423003

git-svn-id: svn://svn.chromium.org/blink/trunk@180987 bbb929c8-8fbe-4397-9dbb-9b2b20218538