Move GCTracer to separate files.

- No functional changes.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/416543003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22600 ce2b1a6d-e550-0410-aec6-3dcde31c8c00