Add UMA metric to track parallel download requests stats.

content/browser/download/download_create_info.h

Index: content/browser/download/download_create_info.h
diff --git a/content/browser/download/download_create_info.h b/content/browser/download/download_create_info.h
index f5c11ea0a23e5b749fd27edaed9dd04c03ee53b1..da7be51834539b96e15e0e4fc7d70440ed04f483 100644
--- a/content/browser/download/download_create_info.h
+++ b/content/browser/download/download_create_info.h
@@ -69,10 +69,6 @@ struct CONTENT_EXPORT DownloadCreateInfo {
   // TODO(xingliu): Refactor to remove |offset| and |length|.
   int64_t offset;
 
-  // The length of the initial request.
-  // This value matches the length in DownloadSaveInfo.
-  int64_t length;
-
   // True if the download was initiated by user action.
   bool has_user_gesture;