Teach benchmark runner to understand generic traces.

BUG=406405
LOG=n
TEST=python -m unittest run_benchmarks_test
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/502473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23329 ce2b1a6d-e550-0410-aec6-3dcde31c8c00