Support for browser arguments was added to run-benchmark in this commit: https://github.com/WebKit/WebKit/commit/fa4a5245bfbbbbd12c6fd9fea70b4efc83f2023a

But only for mac environments!

I'd like linux support, allowing:
```
./run-benchmark  --plan webkitpy/benchmark_runner/data/plans/jetstream.plan --platform linux --browser chrome --browser-args --force-device-scale-factor=.75 --incognito
```

Where `--force-device-scale-factor=.75` and `--incognito` are passed through to the browser.


Also I'd like to be able to use the same argument in the wrapper `browserperfdash-benchmark`. Essentially allowing the following:

```
Tools/Scripts/browserperfdash-benchmark --browser chrome --browser-version 111111 --timestamp 12345 --plan jetstream --config-file /home/spectranaut/chromium-perf-bot/custom/fake_config.txt --browser-args --force-device-scale-factor=.75 --incognito
```

I have a patch for this that I have been using locally. To do this on linux I had to change "browser_args" from a positional to a not-positional argument.