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

chrome/browser/renderer_host/download_throttling_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/download_throttling_resource_handler.h"
 
+#include "base/logging.h"
 #include "chrome/browser/renderer_host/download_resource_handler.h"
 #include "net/base/io_buffer.h"
 
 DownloadThrottlingResourceHandler::DownloadThrottlingResourceHandler(
     ResourceDispatcherHost* host,
     URLRequest* request,
     const GURL& url,
     int render_process_host_id,
     int render_view_id,
     int request_id,
@@ skipping 47 matching lines @@
   if (download_handler_.get())
     return download_handler_->OnWillRead(request_id, buf, buf_size, min_size);
 
   // We should only have this invoked once, as such we only deal with one
   // tmp buffer.
   DCHECK(!tmp_buffer_.get());
   if (min_size < 0)
     min_size = 1024;
   tmp_buffer_ = new net::IOBuffer(min_size);
   *buf = tmp_buffer_.get();
+  // TODO(willchan): Remove after debugging bug 16371.
+  CHECK((*buf)->data());
   *buf_size = min_size;
   return true;
 }
 
 bool DownloadThrottlingResourceHandler::OnReadCompleted(int request_id,
                                                         int* bytes_read) {
   if (ignore_on_read_complete_) {
     // See comments above definition for details on this.
     ignore_on_read_complete_ = false;
     return true;
@@ skipping 62 matching lines @@
   int buf_size;
   if (download_handler_->OnWillRead(request_id_, &buffer, &buf_size,
                                     tmp_buffer_length_)) {
     CHECK(buf_size >= tmp_buffer_length_);
     memcpy(buffer->data(), tmp_buffer_->data(), tmp_buffer_length_);
     download_handler_->OnReadCompleted(request_id_, &tmp_buffer_length_);
   }
   tmp_buffer_length_ = 0;
   tmp_buffer_ = NULL;
 }