When graphing max-rss, we should convert to bytes.

Currently for e.g. 400MB max RSS the graph shows 400k (with a Kilobytes unit).
Another example is `servo-script-*` where the scale stops at 8M (meaning 8GB).

This confused me at first, and IMO it would be easier to read if the unit was bytes.
We don't need to change how we store the value (if that would cause issues), just how we display it.