Fix responsiveness of high promotion mode heuristics.

Fix responsiveness of high promotion mode heuristics.

This changes the heuristics for the high promotion mode to be checked
on all GCs (including scavenges), so that changes in the applications
behavior are recognized earlier. It also fixes miscalculations in the
survival counter and rate which caused heuristics to fail completely.

R=vegorov@chromium.org
BUG=v8:1899

Review URL: https://chromiumcodereview.appspot.com/9196003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10500 ce2b1a6d-e550-0410-aec6-3dcde31c8c00