Add support for serializing/deserializing of SkDrawable

include/core/SkDrawable.h

Index: include/core/SkDrawable.h
diff --git a/include/core/SkDrawable.h b/include/core/SkDrawable.h
index 2f0a62d8b5eb8c824a8c91927fa52a6ecd7b5f5a..6fec3fcf939854080fa2ef3a9953a0fb03e8f879 100644
--- a/include/core/SkDrawable.h
+++ b/include/core/SkDrawable.h
@@ -8,9 +8,10 @@
 #ifndef SkDrawable_DEFINED
 #define SkDrawable_DEFINED
 
-#include "SkRefCnt.h"
+#include "SkFlattenable.h"
 
 class SkCanvas;
+class SkMatrix;
 class SkPicture;
 struct SkRect;
 
@@ -21,7 +22,7 @@ struct SkRect;
  *  allow for clients of the drawable that may want to cache the results, the drawable must
  *  change its generation ID whenever its internal state changes such that it will draw differently.
  */
-class SkDrawable : public SkRefCnt {
+class SkDrawable : public SkFlattenable {
 public:
     SkDrawable();
 
@@ -58,6 +59,9 @@ public:
      */
     void notifyDrawingChanged();
 
+    SK_DEFINE_FLATTENABLE_TYPE(SkDrawable)
+    Factory getFactory() const override { return nullptr; }
+
 protected:
     virtual SkRect onGetBounds() = 0;
     virtual void onDraw(SkCanvas*) = 0;