PartitionAlloc: improve partitionSetNewActivePage.

The improvements are:
- Make the API simpler, leading to simplifications around the two callers.
- More importantly, make it less of a footgun. It now no longer leaves the
page chain in an inconsistent state, which keeps happening to me in a
different CL I am working on. Before, it would tag full pages but leave them
linked in, leading to severe trouble if tagged full pages were scanned over
again later.
- Slightly stronger asserting.

No functional change.

Review URL: https://codereview.chromium.org/131263002

git-svn-id: svn://svn.chromium.org/blink/trunk@164799 bbb929c8-8fbe-4397-9dbb-9b2b20218538