Track size of incremental marking steps in GCTracer.

- Track size of bytes marked incrementally
- Add method to compute marking speed in bytes/millisecond
- Distinguish between cumulative and differential marking stats.
- Fix methods that compute incremental marking stats.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/420213002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22649 ce2b1a6d-e550-0410-aec6-3dcde31c8c00