DM: SKP source / PDF backend

Removed expectations code for PDF backend for now, given that we don't have any, and refactored a little to make that cleaner.

We can now test .skp -> .pdf -> .png in DM.  Neat eh?

BUG=skia:2598
R=halcanary@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/316643003
12 files changed