Use ByteStream in downloads system to decouple source and sink.

net/url_request/url_request_test_job.h

Index: net/url_request/url_request_test_job.h
diff --git a/net/url_request/url_request_test_job.h b/net/url_request/url_request_test_job.h
index 81e41be6877618049050e8e9c7042cb3188e3ebd..a68dd94069166d046a4294e6fea819d553cbe0ce 100644
--- a/net/url_request/url_request_test_job.h
+++ b/net/url_request/url_request_test_job.h
@@ -104,6 +104,12 @@ class NET_EXPORT_PRIVATE URLRequestTestJob : public URLRequestJob {
                                   int* http_status_code) OVERRIDE;
 
  protected:
+  // Override to provide a second round of IO pending after the first
+  // set of data has been retrieved.  This can be used to (for example)
+  // deliver some data but delay completion of a request.
+  // By default the final completion will not be delayed.
+  virtual bool FinalRoundIOPending();
+
   // This is what operation we are going to do next when this job is handled.
   // When the stage is DONE, this job will not be put on the queue.
   enum Stage { WAITING, DATA_AVAILABLE, ALL_DATA, DONE };