cache to 5 (down from 8). Generations 5, 6, and 7 are 
rarely hit in real browsing sessions (~2% in the best case)
so it seems better to lower the size to avoid carrying around 
too many compiled scripts in the cache.
Review URL: http://codereview.chromium.org/119301

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2116 ce2b1a6d-e550-0410-aec6-3dcde31c8c00