runtime: make gcSetTriggerRatio a method of gcControllerState

gcSetTriggerRatio's purpose is to set a bunch of downstream values when
we choose to commit to a new trigger ratio computed by the gcController.
Now that almost all the inputs it uses to compute the downstream values
are in gcControllerState anyway, make it a method of gcControllerState.

For #44167.

Change-Id: I1b7ea709e8378566f812ae3450ab169d7fb66aea
Reviewed-on: https://go-review.googlesource.com/c/go/+/306599


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>