[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

Committed: https://crrev.com/660d4fa7b0a8c487e163d1dd1c76a83a7e31f802
Cr-Commit-Position: refs/heads/master@{#30965}

[Michael Lippautz, 2015-09-28 08:01:43]

mlippautz@chromium.org changed reviewers:
+ hpayer@chromium.org

[Michael Lippautz, 2015-09-28 08:01:56]

The CQ bit was checked by mlippautz@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-28 08:02:15]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371923002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371923002/1

[Hannes Payer (out of office), 2015-09-28 08:22:17]

lgtm

[commit-bot: I haz the power, 2015-09-28 08:27:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-28 08:27:04]

Dry run: This issue passed the CQ dry run.

[Michael Lippautz, 2015-09-28 08:28:01]

The CQ bit was checked by mlippautz@chromium.org

[commit-bot: I haz the power, 2015-09-28 08:28:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371923002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371923002/1

[commit-bot: I haz the power, 2015-09-28 08:32:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 08:35:55]

Patchset 1 (id:??) landed as
https://crrev.com/660d4fa7b0a8c487e163d1dd1c76a83a7e31f802
Cr-Commit-Position: refs/heads/master@{#30965}