SkPdf: SkPDFFormXObject de-class-ified.

We don't need an object, just a few standard fields on the
base class; the change lets us get rid of a bunch of
boilerplate code.

I think this also reduces the cognitive load of the SkPDF
internals.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2185803003

Review-Url: https://codereview.chromium.org/2185803003