Currently, the pacer assumes the goal growth ratio is always exactly
GOGC/100. But sometimes this isn't the case, like when the heap is
very small (limited by heapminimum). So to placate the pacer, we lie
about the value of heap_marked in such situations.

Right now, these two lies make a truth, but GOGC is about to get more
complicated with the introduction of heap limits.

Rather than introduce more lies into the system to handle this,
introduce the concept of an "effective GOGC", which is the GOGC we're
actually using for pacing (we'll need this concept anyway with heap
limits). This commit changes the pacer to use the effective GOGC
rather than the user-set GOGC. This way, we no longer need to lie
about heap_marked because its true value is incorporated into the
effective GOGC.

Change-Id: I5b005258f937ab184ffcb5e76053abd798d542bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/66092
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>