PartitionAlloc: increase alignment in Debug builds again.

The minimum alignment that our Debug cookies imply is now 16. This caters for
XMM instructions on Intel.

Review URL: https://codereview.chromium.org/141703031

git-svn-id: svn://svn.chromium.org/blink/trunk@166560 bbb929c8-8fbe-4397-9dbb-9b2b20218538