Separate output files for --trace-hydrogen.

The output filenames have now the form "hydrogen-<ProcessId>-<IsolateId>.cfg".
Minor cleanup on the way.

Note that we have a similar bug regarding statistics, but this will be handled
in a separate CL.

BUG=v8:2563

Review URL: https://codereview.chromium.org/12450003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13834 ce2b1a6d-e550-0410-aec6-3dcde31c8c00