Add new benchmark suite runner.

TEST=./tools/run_benchmarks.py benchmarks/v8.json
TEST=cd tools/unittests; python -m unittest run_benchmarks_test

Does not support custom results processors yet. Will implement that in a future CL.

BUG=374740
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/293023006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21570 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge