tSweepTerm and pauseStart are supposed to be when STW was triggered,
but right now they're captured a bit before STW. Move these down to
immediately before we trigger STW.

Fixes #19590.

Change-Id: Icd48a5c4d45c9b36187ff986e4f178b5064556c1
Reviewed-on: https://go-review.googlesource.com/49612
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>