gsutil_util: replace `du` with `stat`

`gsutil du PATH` no longer returns an error even if the file does not
exist. Use `gsutil stat` instead.

BUG=chromium:434029
TEST=Manually run the commands using different gsutil versions

Change-Id: I7af64735a2b6dae67fb2b724792c1734da3f3511
Reviewed-on: https://chromium-review.googlesource.com/230305
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
2 files changed