PartitionAlloc: store the root pointer in the super page metadata area.

It enables an additional validity check, and enables further simplifications
later in the patch series such as removing the debug-only pointer from bucket
to root.

BUG=327095

Review URL: https://codereview.chromium.org/129583002

git-svn-id: svn://svn.chromium.org/blink/trunk@164736 bbb929c8-8fbe-4397-9dbb-9b2b20218538