[heap] Adjust number of parallel compaction tasks

[heap] Adjust number of parallel compaction tasks

This is a trivial spinoff of the more complicated CL splitting up memory:
  https://codereview.chromium.org/1365743003/

- Parallel compaction is still off.
- We now compute the number of parallel compaction tasks, depending on the
  evacuation candidate list, the number of cores, and some hard limit.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1371923002

Cr-Commit-Position: refs/heads/master@{#30965}