PartitionAlloc: cleanup, remove root pointer from bucket.

This is another piece extracted from already reviewed but reverted CL:
https://codereview.chromium.org/105503009/

TBR=tsepez@chromium.org
BUG=327095

Review URL: https://codereview.chromium.org/132343005

git-svn-id: svn://svn.chromium.org/blink/trunk@164805 bbb929c8-8fbe-4397-9dbb-9b2b20218538