Use ByteStream in downloads system to decouple source and sink.

net/url_request/url_request_test_job.cc

Index: net/url_request/url_request_test_job.cc
diff --git a/net/url_request/url_request_test_job.cc b/net/url_request/url_request_test_job.cc
index 4a0ba6e21291ca62a345ff57ca84526eefd59129..94a6bf5a757f53a362ddc9f96acce490fe999982 100644
--- a/net/url_request/url_request_test_job.cc
+++ b/net/url_request/url_request_test_job.cc
@@ -247,6 +247,11 @@ void URLRequestTestJob::ProcessNextOperation() {
         if (!ReadRawData(async_buf_, async_buf_size_, &bytes_read))
           NOTREACHED() << "This should not return false in DATA_AVAILABLE.";
         SetStatus(URLRequestStatus());  // clear the io pending flag
+        if (NextReadAsync()) {
+          // Make all future reads return io pending until the next
+          // ProcessNextOperation().
+          stage_ = WAITING;
+        }
         NotifyReadComplete(bytes_read);
       }
       break;
@@ -265,6 +270,10 @@ void URLRequestTestJob::ProcessNextOperation() {
   }
 }
 
+bool URLRequestTestJob::NextReadAsync() {
+  return false;
+}
+
 void URLRequestTestJob::AdvanceJob() {
   if (auto_advance_) {
     MessageLoop::current()->PostTask(