SkPDF:  fix PDF/A

TODO: get a bot to start testing this.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1864163004

Review URL: https://codereview.chromium.org/1864163004
3 files changed