- Fairly (round-robin) divide available memory upon compaction tasks.
- Ensure an upper limit (of memory) since dividing is O(n) for n free-space
  nodes.
- Refill from free lists managed by sweeper once a compaction space becomes
  empty.

Assumption for dividing memory: Memory in the free lists is sparse upon starting
compaction (which means that only few nodes are available), except for memory
reducer GCs, which happen in idle time though (so it's less of a problem).

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1382003002

Cr-Commit-Position: refs/heads/master@{#31234}