Use existing constant for media.Sample Duration

No behavior change, but makes code easier to follow

Resolves pion#119

Author: Sean-Der

play-from-disk-h264/main.go

@@ -110,7 +110,7 @@ func main() { //nolint
 					panic(h264Err)
 				}
 
-				if h264Err = videoTrack.WriteSample(media.Sample{Data: nal.Data, Duration: time.Second}); h264Err != nil {
+				if h264Err = videoTrack.WriteSample(media.Sample{Data: nal.Data, Duration: h264FrameDuration}); h264Err != nil {
 					panic(h264Err)
 				}
 			}