Use conservative estimate for GC speed instead of bailing out when computing

Use conservative estimate for GC speed instead of bailing out when computing
mutator utilization.

This makes GC heuristics more robust for small applications that trigger
scavenges but do not trigger full GC.

BUG=

Committed: https://crrev.com/b528d0772bba9bc378c9cc18f624513ccfc7f8e8
Cr-Commit-Position: refs/heads/master@{#30060}

[ulan, 2015-08-06 16:27:38]

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

[ulan, 2015-08-06 16:27:39]

PTAL

[Hannes Payer (out of office), 2015-08-07 08:53:56]

LGTM

[ulan, 2015-08-07 08:55:12]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-08-07 08:55:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1280703002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1280703002/1

[commit-bot: I haz the power, 2015-08-07 09:35:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-07 09:35:58]

Patchset 1 (id:??) landed as
https://crrev.com/b528d0772bba9bc378c9cc18f624513ccfc7f8e8
Cr-Commit-Position: refs/heads/master@{#30060}