cmd/pprof: ScaleValue fails to scale value with "gb" unit

https://github.com/golang/go/blob/master/src/cmd/pprof/internal/report/report.go#L1501-L1511

``` go
    case "megabyte", "mb":
        value *= 1024 * 1024
    case "gigabyte", "gb":
        value *= 1024 * 1024
```

Last line should be `value *= 1024 * 1024 * 1024`.
