Simplify TurboFan's c1visualizer file handling.

Still having GetTurboCfgFileName in Isolate is ugly, but if we decide that we
don't want to truncate the output file (which would be consistent with
--trace-hydrogen), this could be moved to TurboCfgFile where it actually
belongs.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/666223003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24819 ce2b1a6d-e550-0410-aec6-3dcde31c8c00