Add Context cancellation to clock.

common/gcloud/pubsub/subscriber/subscriber.go

Index: common/gcloud/pubsub/subscriber/subscriber.go
diff --git a/common/gcloud/pubsub/subscriber/subscriber.go b/common/gcloud/pubsub/subscriber/subscriber.go
index 53300d8594ceb362a786edaf8bfdb0e3869dcb76..8a6d1cd0150d80465efff2e7548814c83894e850 100644
--- a/common/gcloud/pubsub/subscriber/subscriber.go
+++ b/common/gcloud/pubsub/subscriber/subscriber.go
@@ -164,18 +164,5 @@ func (s *Subscriber) noDataSleep(c context.Context) {
 	if d <= 0 {
 		d = DefaultNoDataDelay
 	}
-	cancellableSleep(c, d)
-}
-
-// cancellableSleep sleeps, returning either when the sleep duration has expired
-// or the supplied context has been canceled.
-func cancellableSleep(c context.Context, delay time.Duration) {
-	// Sleep for "delay", stopping early if our Context is canceled.
-	select {
-	case <-clock.After(c, delay):
-		break
-
-	case <-c.Done():
-		break
-	}
+	clock.CancelSleep(c, d)
 }