experimental/xps_to_png: fix DPI, usage message

NOTRY=true
Change-Id: I4d4c46b5889ab8e8b8f54e8c631b3932912051dd
Reviewed-on: https://skia-review.googlesource.com/3534
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed