Make GCTracer persistent.

This is essentially https://codereview.chromium.org/390823003/. It turns
out that it's safe to use a persistent GCTracer instead
of creating and destroying GCTracers for each GC, because
Scavenge and Mark-Sweep steps never overlap, and the tracking of
incremental marking also happens separately for the two types of GC.
Tracking of incremental marking will be moved into the GCTracer in a
follow-up patch.

R=hpayer@chromium.org, mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/403543002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22465 ce2b1a6d-e550-0410-aec6-3dcde31c8c00