SkPDF: SkPDFStream takes only SkStreamAsset

SkPDF: SkPDFStream takes only SkStreamAsset

This is possible since https://crrev.com/869763002. Later, I'll clean
up the logic and class constructors.  For now, I assert everywhere I
expect a SkStreamAsset to be well-behaved (duplcate, hasLength).
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2098393002

Review-Url: https://codereview.chromium.org/2098393002