Cleanup remaining class/struct fwd declarations

mojo/public/cpp/bindings/lib/native_struct_data.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_STRUCT_DATA_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_STRUCT_DATA_H_
 
 #include <vector>
 
 #include "mojo/public/cpp/bindings/bindings_export.h"
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
 #include "mojo/public/cpp/system/handle.h"
 
 namespace mojo {
 namespace internal {
 
-class Buffer;
 class ValidationContext;
 
 class MOJO_CPP_BINDINGS_EXPORT NativeStruct_Data {
  public:
   static bool Validate(const void* data, ValidationContext* validation_context);
 
   // Unlike normal structs, the memory layout is exactly the same as an array
   // of uint8_t.
   Array_Data<uint8_t> data;
 
  private:
   NativeStruct_Data() = delete;
   ~NativeStruct_Data() = delete;
 };
 
 static_assert(sizeof(Array_Data<uint8_t>) == sizeof(NativeStruct_Data),
               "Mismatched NativeStruct_Data and Array_Data<uint8_t> size");
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_NATIVE_STRUCT_DATA_H_