Idle notification handler might not free more memory if there was a global GC before or incremental marking is not activated (and it's not worth activating).

BUG=
TEST=cctest/test-api/IdleNotificationWithHint

Review URL: http://codereview.chromium.org/9173007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10401 ce2b1a6d-e550-0410-aec6-3dcde31c8c00