[mojo] Use base::Pickle for Message storage.

mojo/public/cpp/bindings/lib/fixed_buffer.cc

 // Copyright 2014 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.
 
 #include "mojo/public/cpp/bindings/lib/fixed_buffer.h"
 
 #include <stdlib.h>
 
 #include <algorithm>
 
@@ skipping 20 matching lines @@
     MOJO_DCHECK(false) << "Not reached";
     return nullptr;
   }
 
   char* result = ptr_ + cursor_;
   cursor_ += delta;
 
   return result;
 }
 
+PickleBuffer* FixedBuffer::AsPickleBuffer() { return nullptr; }
+
 FixedBufferForTesting::FixedBufferForTesting(size_t size) {
   size_ = internal::Align(size);
   // Use calloc here to ensure all message memory is zero'd out.
   ptr_ = static_cast<char*>(calloc(size_, 1));
 }
 
 FixedBufferForTesting::~FixedBufferForTesting() {
   free(ptr_);
 }
 
 void* FixedBufferForTesting::Leak() {
   char* ptr = ptr_;
   ptr_ = nullptr;
   cursor_ = 0;
   size_ = 0;
   return ptr;
 }
 
 }  // namespace internal
 }  // namespace mojo