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

chrome/browser/renderer_host/download_resource_handler.cc

Index: chrome/browser/renderer_host/download_resource_handler.cc
diff --git a/chrome/browser/renderer_host/download_resource_handler.cc b/chrome/browser/renderer_host/download_resource_handler.cc
index b100df3644ba3380065252d26e40e516e61de7b9..666166668c37f1b3196190f2d376b53923282a66 100644
--- a/chrome/browser/renderer_host/download_resource_handler.cc
+++ b/chrome/browser/renderer_host/download_resource_handler.cc
@@ -4,6 +4,7 @@
 
 #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"
@@ -83,6 +84,8 @@ bool DownloadResourceHandler::OnWillRead(int request_id, net::IOBuffer** buf,
     read_buffer_ = new net::IOBuffer(*buf_size);
   }
   *buf = read_buffer_.get();
+  // TODO(willchan): Remove after debugging bug 16371.
+  CHECK(read_buffer_->data());
   return true;
 }