More aggressive page purging

[Justin Lebar]

>> In summary, I don't think there's a problem here to fix.  Am I missing
>> something?
>
> We could use RSS - number of madvised pages, for sure, but that doesn't
> quite help with people looking at their task manager ans seeing memory
> usage 1GB higher than what it actually is. (And it doesn't help to make
> the numbers Firefox itself reports believable)

Yeah, the perception problem ("I looked at the task manager and
Firefox sux!") is a real one that, from MemShrink's perspective, we
need to solve.

But also, |RSS minus madvised memory| is wrong as soon as any of the
madvised memory is kicked out due to memory pressure.  So without
double-purge semantics, I don't see how you'd be able to reliably
measure "hard" memory usage.

-Justin