SkPDF Fix PDF/A to be more valid

Some produced PDFs are recognized by veraPDF 0.13.2
(http://verapdf.org/software/) as valid PDF/A-2B.

BUG=skia:4535

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1864173004

Review URL: https://codereview.chromium.org/1864173004
2 files changed