Make sure that memory reducer makes progress in incremental marking

Make sure that memory reducer makes progress in incremental marking
even if there are no idle notifications.

BUG=chromium:515873
LOG=NO

Committed: https://crrev.com/1345f811bf6a3ab9376cfbed6f08084662b32a4d
Cr-Commit-Position: refs/heads/master@{#30062}

[ulan, 2015-08-06 13:09:01]

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

[ulan, 2015-08-06 13:09:01]

PTAL

[Hannes Payer (out of office), 2015-08-07 05:13:31]

On 2015/08/06 13:09:01, ulan wrote:
> PTAL

Lgtm

[ulan, 2015-08-07 09:55:36]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-08-07 09:55:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1274633003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1274633003/20001

[commit-bot: I haz the power, 2015-08-07 10:20:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-07 10:21:17]

Patchset 2 (id:??) landed as
https://crrev.com/1345f811bf6a3ab9376cfbed6f08084662b32a4d
Cr-Commit-Position: refs/heads/master@{#30062}