Remove non-incremental GC from memory reducer.

Remove non-incremental GC from memory reducer.

Incremental GC now finish quickly in background tabs too because they
do not rely on idle notifications.

BUG=

Committed: https://crrev.com/5449c9848aac7ccf79f440a269bcd5d3865b0738
Cr-Commit-Position: refs/heads/master@{#31461}

[ulan, 2015-10-21 14:43:32]

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

[ulan, 2015-10-21 14:43:32]

PTAL

[Hannes Payer (out of office), 2015-10-22 09:23:58]

lgtm

[ulan, 2015-10-22 09:24:31]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-10-22 09:24:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1423453003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1423453003/1

[commit-bot: I haz the power, 2015-10-22 10:23:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-22 10:24:07]

Patchset 1 (id:??) landed as
https://crrev.com/5449c9848aac7ccf79f440a269bcd5d3865b0738
Cr-Commit-Position: refs/heads/master@{#31461}