net/http: define File as an io.Seeker

Fixes #13421.

Change-Id: Ic0fc0d0a8e86d2be23db12ddd1a8a8105e26ee56
Reviewed-on: https://go-review.googlesource.com/17250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/fs.go

@@ -63,8 +63,8 @@ type FileSystem interface {
 type File interface {
 	io.Closer
 	io.Reader
+	io.Seeker
 	Readdir(count int) ([]os.FileInfo, error)
-	Seek(offset int64, whence int) (int64, error)
 	Stat() (os.FileInfo, error)
 }