Fix bug with WaitForArtifacts that has a side-effect that didn't matter before.

Async broke the assumption that WaitForArtifacts would only be called once.
Make the side-effect optional.

BUG=chromium:274824
TEST=Ran stage of deltas with async=True and is_synced etc + unittests.

Change-Id: I012385fe1cbeadffb7ae283051aad502576a7325
Reviewed-on: https://gerrit.chromium.org/gerrit/66149
Reviewed-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
diff --git a/downloader.py b/downloader.py
index e8e67f0..68192f6 100755
--- a/downloader.py
+++ b/downloader.py
@@ -131,7 +131,7 @@
         # thread. This prevents caller from waiting indefinitely for any
         # nonexistent artifact.
         for artifact in required_artifacts:
-          artifact.WaitForArtifactToExist(timeout=10)
+          artifact.WaitForArtifactToExist(timeout=10, update_name=False)
         self._DownloadArtifactsInBackground(required_artifacts)
       else:
         self._DownloadArtifactsSerially(required_artifacts, no_wait=True)