Upload multiple dm images at once

Use the -m file, as recommended in
https://luci-milo.appspot.com/swarming/task/3b197fbd667a1510/steps/upload_images/0/stdout

Bug: skia:
Change-Id: I58d101f58594bf221d11a787057ee7950b197f86
Reviewed-on: https://skia-review.googlesource.com/94965
Reviewed-by: Stephan Altmueller <stephana@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
12 files changed