Add CHECKs to the ResourceHandler derived classes to see which is returning a NULL IOBuffer:data_.

chrome/browser/renderer_host/save_file_resource_handler.cc

 // Copyright (c) 2006-2008 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 "chrome/browser/renderer_host/save_file_resource_handler.h"
 
+#include "base/logging.h"
 #include "base/message_loop.h"
 #include "base/string_util.h"
 #include "chrome/browser/download/save_file_manager.h"
 #include "net/base/io_buffer.h"
 
 SaveFileResourceHandler::SaveFileResourceHandler(int render_process_host_id,
                                                  int render_view_id,
                                                  const GURL& url,
                                                  SaveFileManager* manager)
     : save_id_(-1),
@@ skipping 32 matching lines @@
 }
 
 bool SaveFileResourceHandler::OnWillRead(int request_id, net::IOBuffer** buf,
                                          int* buf_size, int min_size) {
   DCHECK(buf && buf_size);
   if (!read_buffer_) {
     *buf_size = min_size < 0 ? kReadBufSize : min_size;
     read_buffer_ = new net::IOBuffer(*buf_size);
   }
   *buf = read_buffer_.get();
+  // TODO(willchan): Remove after debugging bug 16371.
+  CHECK(read_buffer_->data());
   return true;
 }
 
 bool SaveFileResourceHandler::OnReadCompleted(int request_id, int* bytes_read) {
   DCHECK(read_buffer_);
   // We are passing ownership of this buffer to the save file manager.
   net::IOBuffer* buffer = NULL;
   read_buffer_.swap(&buffer);
   save_manager_->GetSaveLoop()->PostTask(FROM_HERE,
       NewRunnableMethod(save_manager_,
@@ skipping 16 matching lines @@
                         render_process_id_,
                         status.is_success() && !status.is_io_pending()));
   read_buffer_ = NULL;
   return true;
 }
 
 void SaveFileResourceHandler::set_content_length(
     const std::string& content_length) {
   content_length_ = StringToInt64(content_length);
 }