Before this patch all tracing scopes in incremental marking would be reset
during a gc tracer start/stop cycle. This patch handles scopes the same way it
does other incremental marking metrics.

Also:
- Align finalization metric with regular marking metric.
- Smaller cleanups

BUG=chromium:639818
R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2264033002
Cr-Commit-Position: refs/heads/master@{#38822}