SkPDF: even more PDFIndirectRefs

  * SkPDFUnion.h hidden.
  * SkPDFObjectSerializer::serializeObject gone.
  * SkPDFDocument::serialize gone
  * SkPDFMetadata::MakeXMPObject return PDFIndirectRef
  * SkPDFTag rewritten from scratch.
  * SkPDFObject::addResources virtual gone.
  * SkPDFObject::drop virtual gone.
  * SkPDFArray::appendObjRef gone.
  * SkPDFDict::insertObjRef gone.
  * SkPDFDict::Record becomes std::pair
  * SkPDFObjNumMap gone;
  * SkPDFObject::fIndirectReference gone;

Bug: skia:8630
Change-Id: I7425eef30548b54f10fbfe96dd502800cb150caf
Reviewed-on: https://skia-review.googlesource.com/c/178935
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
15 files changed