godoc/dl: add dmitshur, katiehockman to binary upload whitelist

This is for uploading Go release binaries to dl.google.com with
x/build/cmd/release.

Updates golang/go#27953.

Change-Id: Idc9e5d5c4cf4c1e5602b51cc1159c982038c7901
Reviewed-on: https://go-review.googlesource.com/138879
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

godoc/dl/dl.go

@@ -480,7 +480,7 @@ Nothing to see here; <a href="https://golang.org/dl/#%s">move along</a>.
 
 func validUser(user string) bool {
 	switch user {
-	case "adg", "bradfitz", "cbro", "andybons", "valsorda":
+	case "adg", "bradfitz", "cbro", "andybons", "valsorda", "dmitshur", "katiehockman":
 		return true
 	}
 	return false