- Wasted bytes are now accounted where they accrue, i.e., the corresponding free
  list. The amount of waste is transferred by concatenating free lists.
- During concatenation, free lists are no longer locked on FreeListCategory
  level, but in the FreeList itself, simplifying the sync between contained nodes
  and wasted bytes (which are effectively dropped nodes).

This is pre-work for properly moving memory to compaction spaces, which requires
correct accounting of wasted memory.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1379833002

Cr-Commit-Position: refs/heads/master@{#31040}