Add a test to check partitionAlloc's immediate-double-free detection.

Split out from https://codereview.chromium.org/119843002/ (already LGTM'ed) for
good CL hygiene.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/103253007

git-svn-id: svn://svn.chromium.org/blink/trunk@164265 bbb929c8-8fbe-4397-9dbb-9b2b20218538