Do full GC for small heaps in idle time handler.

BUG=407593
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/563573002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23871 ce2b1a6d-e550-0410-aec6-3dcde31c8c00