windows/svc: add Context to ChangeRequest

New Context field will be used in the following CL to test
ctlHandler parameter alignments.

Also adjust TestExample to pass hard coded Context value of 123456
to test service, and verify that correct value is logged. Final
part of the test is commented out, and will be adjusted in the next
CL.

Updates golang/go#25660

Change-Id: Iad2896ae497ee1edc0d62655eaf08671ec2651c5
Reviewed-on: https://go-review.googlesource.com/c/158697
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/svc/example/service.go

@@ -27,7 +27,6 @@ func (m *myservice) Execute(args []string, r <-chan svc.ChangeRequest, changes c
 	slowtick := time.Tick(2 * time.Second)
 	tick := fasttick
 	changes <- svc.Status{State: svc.Running, Accepts: cmdsAccepted}
-	elog.Info(1, strings.Join(args, "-"))
 loop:
 	for {
 		select {
@@ -42,6 +41,10 @@ loop:
 				time.Sleep(100 * time.Millisecond)
 				changes <- c.CurrentStatus
 			case svc.Stop, svc.Shutdown:
+				// golang.org/x/sys/windows/svc.TestExample is verifying this output.
+				testOutput := strings.Join(args, "-")
+				testOutput += fmt.Sprintf("-%d", c.Context)
+				elog.Info(1, testOutput)
 				break loop
 			case svc.Pause:
 				changes <- svc.Status{State: svc.Paused, Accepts: cmdsAccepted}