v8: GCTracer clean-up part 1.

v8: GCTracer clean-up part 1.

- Split GCTracer::~GCTracer into printing functions and update of variables
tracked on Heap.
- Clean-up recording of time, object size and memory size at the beginning and
end of GC.
- Consistently use enum notation in ScopeId.
- Disallow copy and assign for Scope and GCTracer.
- Remove unused stats on GCTracer.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/396893004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00