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

chrome/browser/renderer_host/buffered_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/buffered_resource_handler.h"
 
 #include "base/histogram.h"
+#include "base/logging.h"
 #include "base/string_util.h"
 #include "net/base/mime_sniffer.h"
 #include "net/base/net_errors.h"
 #include "chrome/browser/renderer_host/download_throttling_resource_handler.h"
 #include "chrome/browser/renderer_host/resource_dispatcher_host.h"
 #include "chrome/common/url_constants.h"
 #include "net/base/mime_sniffer.h"
 #include "net/base/io_buffer.h"
 #include "net/http/http_response_headers.h"
 
@@ skipping 63 matching lines @@
 
 // We'll let the original event handler provide a buffer, and reuse it for
 // subsequent reads until we're done buffering.
 bool BufferedResourceHandler::OnWillRead(int request_id, net::IOBuffer** buf,
                                          int* buf_size, int min_size) {
   if (buffering_) {
     DCHECK(!my_buffer_.get());
     my_buffer_ = new net::IOBuffer(kMaxBytesToSniff);
     *buf = my_buffer_.get();
     *buf_size = kMaxBytesToSniff;
+    // TODO(willchan): Remove after debugging bug 16371.
+    CHECK((*buf)->data());
     return true;
   }
 
   if (finished_)
     return false;
 
   if (!real_handler_->OnWillRead(request_id, buf, buf_size, min_size)) {
     return false;
   }
   read_buffer_ = *buf;
+  // TODO(willchan): Remove after debugging bug 16371.
+  CHECK(read_buffer_->data());
   read_buffer_size_ = *buf_size;
   DCHECK(read_buffer_size_ >= kMaxBytesToSniff * 2);
   bytes_read_ = 0;
   return true;
 }
 
 bool BufferedResourceHandler::OnReadCompleted(int request_id, int* bytes_read) {
   if (sniff_content_ || should_buffer_) {
     if (KeepBuffering(*bytes_read))
       return true;
@@ skipping 172 matching lines @@
     real_handler_ = download_handler;
   }
   return real_handler_->OnResponseStarted(request_id, response_);
 }
 
 bool BufferedResourceHandler::DidBufferEnough(int bytes_read) {
   const int kRequiredLength = 256;
 
   return bytes_read >= kRequiredLength;
 }