Move the SuperPageBitmap from PageAlloctor to PartitionAlloc.

No functional change. We're about to properly genericize PageAllocator, so this
move is necessary in the short term to avoid a layering violation.

BUG=324960

Review URL: https://codereview.chromium.org/99323006

git-svn-id: svn://svn.chromium.org/blink/trunk@163042 bbb929c8-8fbe-4397-9dbb-9b2b20218538