Use ByteStream in downloads system to decouple source and sink.

net/base/net_log_event_type_list.h

Index: net/base/net_log_event_type_list.h
diff --git a/net/base/net_log_event_type_list.h b/net/base/net_log_event_type_list.h
index beb45176f891ce633f5b303c7e934924ecc73c8c..04862e2a257c13803b893fc4c207894f6699e1b3 100644
--- a/net/base/net_log_event_type_list.h
+++ b/net/base/net_log_event_type_list.h
@@ -1583,11 +1583,13 @@ EVENT_TYPE(DOWNLOAD_ITEM_CANCELED)
 //   }
 EVENT_TYPE(DOWNLOAD_FILE_OPENED)
 
-// This event is created when a download file is written to.
+// This event is created when the stream between download source
+// and download file is drained.
 //   {
-//     "byte_count": <Number of bytes written in this call>,
+//     "stream_size": <Total size of all bytes drained from the stream>
+//     "num_buffers": <How many separate buffers those bytes were in>
 //   }
-EVENT_TYPE(DOWNLOAD_FILE_WRITTEN)
+EVENT_TYPE(DOWNLOAD_STREAM_DRAINED)
 
 // This event is created when a download file is renamed.
 //   {