Add event statistics to GCTracer.

- Add method to GCTracer to compute mean and max of the last few
  Scavenger, Mark Compactor and Incremental Marking events.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/410413005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22605 ce2b1a6d-e550-0410-aec6-3dcde31c8c00