Before this CL, free memory (FreeSpace) has been managed through a global free
list that contains single-linked lists of FreeSpace nodes for each size class.

We move away from this approach to a global two-level doubly-linked list that
refers to singly-linked lists of FreeSpace nodes on the corresponding pages.
This way we can refill on a page-level granularity. Furthermore, it also enables
constant-time eviction of pages from the free list.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1772733002

Cr-Commit-Position: refs/heads/master@{#34853}