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

chrome/browser/renderer_host/cross_site_resource_handler.cc

 // Copyright (c) 2009 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 <string>
 
 #include "chrome/browser/renderer_host/cross_site_resource_handler.h"
 
+#include "base/logging.h"
 #include "base/message_loop.h"
 #include "chrome/browser/renderer_host/render_view_host.h"
+#include "net/base/io_buffer.h"
 
 namespace {
 
 // Task to notify the TabContents that a cross-site response has begun, so that
 // TabContents can tell the old page to run its onunload handler.
 class CrossSiteNotifyTask : public Task {
  public:
   CrossSiteNotifyTask(int render_process_host_id,
                       int render_view_id,
                       int request_id)
@@ skipping 95 matching lines @@
   }
 
   // Tell the renderer to run the onunload event handler, and wait for the
   // reply.
   StartCrossSiteTransition(request_id, response, global_id);
   return true;
 }
 
 bool CrossSiteResourceHandler::OnWillRead(int request_id, net::IOBuffer** buf,
                                           int* buf_size, int min_size) {
-  return next_handler_->OnWillRead(request_id, buf, buf_size, min_size);
+  bool rv = next_handler_->OnWillRead(request_id, buf, buf_size, min_size);
+  // TODO(willchan): Remove after debugging bug 16371.
+  if (rv)
+    CHECK((*buf)->data());
+  return rv;
 }
 
 bool CrossSiteResourceHandler::OnReadCompleted(int request_id,
                                                int* bytes_read) {
   if (!in_cross_site_transition_) {
     return next_handler_->OnReadCompleted(request_id, bytes_read);
   }
   return true;
 }
 
@@ skipping 117 matching lines @@
 
   // Tell the tab responsible for this request that a cross-site response is
   // starting, so that it can tell its old renderer to run its onunload
   // handler now.  We will wait to hear the corresponding ClosePage_ACK.
   CrossSiteNotifyTask* task =
       new CrossSiteNotifyTask(render_process_host_id_,
                               render_view_id_,
                               request_id);
   rdh_->ui_loop()->PostTask(FROM_HERE, task);
 }