[pdf] Have DM set producer

The default PDF producer string includes the Skia milestone number,
making them change every time the milestone is updated. Avoid this when
running DM.

Change-Id: I96dcd18fe0a5c9f7f4fd5c6e092e57b30c0d28ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/864917
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>