SkPDF: add canon assert before adding code that might break it

Motivation: We can write subsets (by page) of pdf documents (but this
in't yet exposed in the public API), but it is a bad idea to mix pages
from multiple documents (de-duping will break).  This assert verifies
that we don't do this by accident in the future.

BUG=skia:3585

Review URL: https://codereview.chromium.org/1037573005
3 files changed