browser_preferences_usage.js hardcodes a max value of 15 accesses which doesn't scale well as we increase the number of processes. Instead we'll base the max on a multiplier of the default process count which should avoid bustage in the future but still catch egregious accesses.

--HG--
extra : rebase_source : 9c047b27b6d7a2666f8889ffcee3754762e40987