Revert "Simplifying GC heuristics, deleted old generation allocation limit."

This reverts commit 14731029.

BUG=

Review URL: https://codereview.chromium.org/14750013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14711 ce2b1a6d-e550-0410-aec6-3dcde31c8c00