Taking this now that we've landed bug 1523974. Most of the remaining code in the process priority manager is being used but there's still some dead bits we need to remove. The most obvious is the handling of the 'high-priority' wakelock: it's a leftover of FxOS and ironically it was already unused in the late FxOS versions so it's as dead as can be. Then there's the 'cpu' wakelock which also seems unused but there's a possibility that Fennec is still using it so I'll have to double-check before cutting it out.