Reapply win gn fixes from
https://codereview.chromium.org/694353003/
https://codereview.chromium.org/697183003/
https://codereview.chromium.org/700763002/

that were undone by
https://codereview.chromium.org/693943003

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/701953004