SkPDF: PDFStream has-a not is-a PDFDict

Motivation:
SkPDFStream and SkPDFSharedStream now work the same.

Also:
- move SkPDFStream into SkPDFTypes (it's a fundamental PDF type).
- minor refactor of SkPDFSharedStream
- SkPDFSharedStream takes unique_ptr to represent ownership

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2190883003

Review-Url: https://codereview.chromium.org/2190883003
16 files changed