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

chrome/browser/renderer_host/sync_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/sync_resource_handler.h"
+#include "base/logging.h"
 #include "chrome/common/render_messages.h"
 
 SyncResourceHandler::SyncResourceHandler(
     ResourceDispatcherHost::Receiver* receiver,
     const GURL& url,
     IPC::Message* result_message)
     : read_buffer_(new net::IOBuffer(kReadBufSize)),
       receiver_(receiver),
       result_message_(result_message) {
   result_.final_url = url;
@@ skipping 20 matching lines @@
   result_.headers = response->response_head.headers;
   result_.mime_type = response->response_head.mime_type;
   result_.charset = response->response_head.charset;
   return true;
 }
 
 bool SyncResourceHandler::OnWillRead(int request_id, net::IOBuffer** buf,
                                      int* buf_size, int min_size) {
   DCHECK(min_size == -1);
   *buf = read_buffer_.get();
+  // TODO(willchan): Remove after debugging bug 16371.
+  CHECK(read_buffer_->data());
   *buf_size = kReadBufSize;
   return true;
 }
 
 bool SyncResourceHandler::OnReadCompleted(int request_id, int* bytes_read) {
   if (!*bytes_read)
     return true;
   result_.data.append(read_buffer_->data(), *bytes_read);
   return true;
 }
 
 bool SyncResourceHandler::OnResponseCompleted(
     int request_id,
     const URLRequestStatus& status,
     const std::string& security_info) {
   result_.status = status;
 
   ViewHostMsg_SyncLoad::WriteReplyParams(result_message_, result_);
   receiver_->Send(result_message_);
   result_message_ = NULL;
   return true;
 }