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

chrome/browser/renderer_host/download_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_resource_handler.h"
 
+#include "base/logging.h"
 #include "chrome/browser/download/download_file.h"
 #include "chrome/browser/download/download_manager.h"
 #include "chrome/browser/renderer_host/resource_dispatcher_host.h"
 #include "net/base/io_buffer.h"
 #include "net/url_request/url_request_context.h"
 
 DownloadResourceHandler::DownloadResourceHandler(ResourceDispatcherHost* rdh,
                                                  int render_process_host_id,
                                                  int render_view_id,
                                                  int request_id,
@@ skipping 59 matching lines @@
 // Create a new buffer, which will be handed to the download thread for file
 // writing and deletion.
 bool DownloadResourceHandler::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;
 }
 
 // Pass the buffer to the download file writer.
 bool DownloadResourceHandler::OnReadCompleted(int request_id, int* bytes_read) {
   if (!*bytes_read)
     return true;
   DCHECK(read_buffer_);
   AutoLock auto_lock(buffer_->lock);
   bool need_update = buffer_->contents.empty();
@@ skipping 70 matching lines @@
                        should_pause);
     is_paused_ = should_pause;
   }
 }
 
 void DownloadResourceHandler::StartPauseTimer() {
   if (!pause_timer_.IsRunning())
     pause_timer_.Start(base::TimeDelta::FromMilliseconds(kThrottleTimeMs), this,
                        &DownloadResourceHandler::CheckWriteProgress);
 }