Adjust small heap threshold for idle time handler.

Follow-up for r23871.

BUG=chromium:407593
LOG=N
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/592053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24116 ce2b1a6d-e550-0410-aec6-3dcde31c8c00