Shrink the per-partition page metadata structure.

The space required per partition page is down to 32 bytes now, on all systems.
This CL realizes the following benefits:

- The allocator metadata load is now 32/16384, or 0.2%. This compares favourably
with tcmalloc, which usually is around 1%.

- The doubly-linked active page ring list is now a singly-linked NULL-terminated
list. Operations that scan over and modify this list are now much simpler and
in some cases cheaper.

BUG=324960

Review URL: https://codereview.chromium.org/108753002

git-svn-id: svn://svn.chromium.org/blink/trunk@163366 bbb929c8-8fbe-4397-9dbb-9b2b20218538