run-perf-tests now generates results page

Have you been annoyed by having to manually copy and paste performance test results from Terminal? I say no more!

I'm adding run-perf-tests the ability to generate results.html with beautiful flot graphs in WebKit bug 92575.

It can even aggregate results from multiple runs of run-perf-tests. So if you've ran run-perf-tests multiple times with the same JSON file path (specified by --output-json-path), results.html will contain the results from all previous runs in each graph.

Furthermore, you can aggregate results from different WebKit checkouts because the output JSON file is solely determined by --output-json-path option.

For example, if you have two checkouts of WebKit with and without a patch, you can execute run-webkit-perf --output-json-path=/Users/rniwa/output.json on both checkouts and compare the results.