This reverts commit 7a0a0b8b.

- The number of parallel tasks is still 1, i.e., we only compact on the main
  thread.
- Remove emergency memory (PagedSpace, and CodeRange)
- Introduce partial compaction of pages.
- Logic for multiple tasks is in place.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1356533002

Cr-Commit-Position: refs/heads/master@{#30796}