Enable flattening/unflattening with custom unflatten procs

include/core/SkWriteBuffer.h

Index: include/core/SkWriteBuffer.h
diff --git a/include/core/SkWriteBuffer.h b/include/core/SkWriteBuffer.h
index 306a3e37a9f27cc6a36ef8642bdb4961656a3629..ba7ac454324b0c9c7117321e4aa6da2f63944651 100644
--- a/include/core/SkWriteBuffer.h
+++ b/include/core/SkWriteBuffer.h
@@ -16,6 +16,7 @@
 #include "SkPixelSerializer.h"
 #include "SkRefCnt.h"
 #include "SkWriter32.h"
+#include "../private/SkTHash.h"
 
 class SkBitmap;
 class SkBitmapHeap;
@@ -118,6 +119,10 @@ private:
     SkRefCntSet* fTFSet;
 
     SkAutoTUnref<SkPixelSerializer> fPixelSerializer;
+
+    // Only used if we do not have an fFactorySet
+    SkTHashMap<SkString, uint32_t> fFlattenableDict;
+    uint32_t fNextIndex;
 };
 
 #endif // SkWriteBuffer_DEFINED