The CompactionSpace is temporarily used during compaction to hold migrated
objects. The payload is merged back into the corresponding space after
compaction.

Note the this is not the complete implementation and it is currently only used in a test.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1314493007

Cr-Commit-Position: refs/heads/master@{#30407}