Move partition page metadata out-of-line.

There are further simplifcations to follow, but with this CL, we realize these
benefits:

- Much less memory waste for certain common bucket sizes. The worst case is
probably 2048 bytes which is a common allocation size and leads to a 12.5%
wastage due to 2048 fitting perfectly in 16384 (partition page size) but us
losing an entire 2048 byte chunk due to the 64 byte metadata header. We've
now ditched that header.

- Complete removal of re-entrancy from the allocator. Now that the metadata is
out-of-line, we don't lose it when freeing system pages. Therefore, we don't
need to allocate any space for it on free. It's always there.

- Minor security improvement. It's now not possible to corrupt metadata with
any linear buffer overflow or underflow.

BUG=324960

Review URL: https://codereview.chromium.org/99143003

git-svn-id: svn://svn.chromium.org/blink/trunk@163253 bbb929c8-8fbe-4397-9dbb-9b2b20218538