This change removes the unswept free bytes counter.

The new approach
- directly decrements allocated memory and capacity before sweeping (using live
  bytes from the marker), and
- adds back capacity during refilling a free list.

This is another pre-work for moving around free lists while keeping the counters
in a sane state.

The previous approach allowed us to nail down how much memory is to-be-swept.
However, there were no users of this as we only used it do decrement it from
allocated memory (which still accounted for dead objects).  If we want to keep
track of unswept free bytes in a space during compaction we can introduce a
separate new concurrent counter for this purpose.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1380723002

Cr-Commit-Position: refs/heads/master@{#31175}