Make verbose.log optional in upload_dm_results

Will land/test after https://skia-review.googlesource.com/c/skia/+/147042

Bug: skia:
Change-Id: I381e369df7fb904c81948f36dbad679d3f430415
Reviewed-on: https://skia-review.googlesource.com/147202
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
10 files changed