Add option to compute average scavenge speed w.r.t survived objects.

Add option to compute average scavenge speed w.r.t survived objects.

Use it in detection of low young generation allocation rate.

BUG=501314
LOG=NO
TBR=hpayer@chromium.org

Committed: https://crrev.com/b62a7a833b80e7408a42b3a7fe93672097d5aaa7
Cr-Commit-Position: refs/heads/master@{#29094}

[ulan, 2015-06-17 14:47:51]

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

[ulan, 2015-06-17 14:47:52]

PTAL. This make scavenge speed more stable and fixes regression in Octane where
low survival rate causes high scavenge speed, which breaks computation of
mutator utilization for new space.

[ulan, 2015-06-17 15:00:42]

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

[commit-bot: I haz the power, 2015-06-17 15:00:55]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1186903005/40001

[commit-bot: I haz the power, 2015-06-17 15:59:25]

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

[commit-bot: I haz the power, 2015-06-17 15:59:25]

Dry run: This issue passed the CQ dry run.

[ulan, 2015-06-17 16:01:15]

On 2015/06/17 15:59:25, commit-bot: I haz the power wrote:
> Dry run: This issue passed the CQ dry run.

Hannes, I'll go ahead and land this with tbr since this fixes perf regression.
I'll address your comments (if any) in a separate CL.

[ulan, 2015-06-17 16:01:22]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-06-17 16:01:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1186903005/40001

[commit-bot: I haz the power, 2015-06-17 16:03:11]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-17 16:03:31]

Patchset 3 (id:??) landed as
https://crrev.com/b62a7a833b80e7408a42b3a7fe93672097d5aaa7
Cr-Commit-Position: refs/heads/master@{#29094}