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

chrome/browser/renderer_host/async_resource_handler.h

Index: chrome/browser/renderer_host/async_resource_handler.h
diff --git a/chrome/browser/renderer_host/async_resource_handler.h b/chrome/browser/renderer_host/async_resource_handler.h
index cfb9fc5fc9ab6e616beef90bb247793093f8ddf2..89ecfad26b4abc24da8a4d0816790a09d0c536aa 100644
--- a/chrome/browser/renderer_host/async_resource_handler.h
+++ b/chrome/browser/renderer_host/async_resource_handler.h
@@ -38,11 +38,6 @@ class AsyncResourceHandler : public ResourceHandler {
   static void GlobalCleanup();
 
  private:
-  // When reading, we don't know if we are going to get EOF (0 bytes read), so
-  // we typically have a buffer that we allocated but did not use.  We keep
-  // this buffer around for the next read as a small optimization.
-  static SharedIOBuffer* spare_read_buffer_;
-
   scoped_refptr<SharedIOBuffer> read_buffer_;
   ResourceDispatcherHost::Receiver* receiver_;
   int process_id_;