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
diff --git a/src/pdf/SkPDFGraphicState.h b/src/pdf/SkPDFGraphicState.h
index 70b63d4..6b3f7cc 100644
--- a/src/pdf/SkPDFGraphicState.h
+++ b/src/pdf/SkPDFGraphicState.h
@@ -9,7 +9,7 @@
 #ifndef SkPDFGraphicState_DEFINED
 #define SkPDFGraphicState_DEFINED
 
-#include "SkPDFStream.h"
+#include "SkPDFTypes.h"
 #include "SkChecksum.h"
 
 class SkPaint;