PartitionAlloc: add an extra double-free detection.

It's basically free because it's in the slow path.

BUG=336359
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/138163010

git-svn-id: svn://svn.chromium.org/blink/trunk@165588 bbb929c8-8fbe-4397-9dbb-9b2b20218538