This change makes it possible to retrieve the size of a file part
without having to Seek to determine file-size.

Resolves #19501

Change-Id: I7b9994c4cf41c9b06a046eb7046f8952ae1f15e9
Reviewed-on: https://go-review.googlesource.com/39223Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>