Enable flattening/unflattening with custom unflatten procs

src/core/SkFlattenableSerialization.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkFlattenableSerialization.h"
 
 #include "SkData.h"
 #include "SkValidatingReadBuffer.h"
 #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);
     writer.writeToMemory(data->writable_data());
     return data.release();
 }
 
 SkFlattenable* SkValidatingDeserializeFlattenable(const void* data, size_t size,
                                                   SkFlattenable::Type type) {
     SkValidatingReadBuffer buffer(data, size);
     return buffer.readFlattenable(type);
 }