Enable flattening/unflattening with custom unflatten procs

src/core/SkFlattenableSerialization.cpp

Index: src/core/SkFlattenableSerialization.cpp
diff --git a/src/core/SkFlattenableSerialization.cpp b/src/core/SkFlattenableSerialization.cpp
index e9ce09ff1f7f8d4b40b960c4f0435cc0cfe9e02f..06e8c1070616c1dc2aa76cf3720389a2abbdef93 100644
--- a/src/core/SkFlattenableSerialization.cpp
+++ b/src/core/SkFlattenableSerialization.cpp
@@ -12,7 +12,7 @@
 #include "SkWriteBuffer.h"
 
 SkData* SkValidatingSerializeFlattenable(SkFlattenable* flattenable) {
-    SkWriteBuffer writer(SkWriteBuffer::kValidation_Flag);
+    SkWriteBuffer writer;
     writer.writeFlattenable(flattenable);
     size_t size = writer.bytesWritten();
     auto data = SkData::MakeUninitialized(size);