SkPDF: SkPDFDevice has ptr to SkPDFDocument

This is necessary for pre-serialization of images.

BUG=skia:5087
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1802963002

Review URL: https://codereview.chromium.org/1802963002
6 files changed