- 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.

This is a re-land of https://codereview.chromium.org/391413006/ with
 - int instead of size_t in ring buffer unit test.
 - git cl format

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/411293002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22593 ce2b1a6d-e550-0410-aec6-3dcde31c8c00