Always trace garbage collection events in GCTracer.

- always trace events in GCTracer, but only print stats when the command
  line flag is set.
- required to collect statistis that will be used to schedue garbage
  collection.
- we need to watch the perf bots after this lands, as it could regress
  performance.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/410413007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22613 ce2b1a6d-e550-0410-aec6-3dcde31c8c00