Get rid of kSubPartitionPage size.

We don't need any concept of a "sub-partition" page. All we care about is system
page boundaries so we can simplify the number of concepts in play.

This CL also fixes an off-by-one bug in partitionPageAllocAndFillFreelist() that
was miscalulating the extent of a system page and causing us to not fill as many
freelist entries as we should. The bug has a much larger impact when we move
page metadata out-of-line, but it makes sense to fix it with this CL.

BUG=324960

Review URL: https://codereview.chromium.org/100663002

git-svn-id: svn://svn.chromium.org/blink/trunk@163053 bbb929c8-8fbe-4397-9dbb-9b2b20218538