- track incremental marking stats directly on GCTracer.
- add simple ring buffer class.
- track last 10 scavenges and mark-compacts in ring buffers on GCTracer.
- various clean-ups.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/391413006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22567 ce2b1a6d-e550-0410-aec6-3dcde31c8c00