Currently callers of gcDrain control whether it flushes scan work
credit to gcController.bgScanCredit by passing a value other than -1
for the flush threshold. Shortly we're going to make this always flush
scan work to gcController.scanWork and optionally also flush scan work
to gcController.bgScanCredit. This will be much easier if the flush
threshold is simply a constant (which it is in practice) and callers
merely control whether or not the flush includes the background
credit. Hence, replace the flush threshold argument with a flag.

Change-Id: Ia27db17de8a3f1e462a5d7137d4b5dc72f99a04e
Reviewed-on: https://go-review.googlesource.com/15406Reviewed-by: Rick Hudson <rlh@golang.org>