Add support in devserver to stage debug_breakpad.tar.xz

BUG=chromium:623739
TEST=unittest, local run devserver
curl "http://localhost:8082/stage?artifacts=symbols_only&files=&archive_url= \
  gs_path_to_build"

Change-Id: I7fbff78a91218b408c3f054a5742b6d887734bdb
Reviewed-on: https://chromium-review.googlesource.com/393008
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/artifact_info.py b/artifact_info.py
index a6ff6f7..4c0484a 100644
--- a/artifact_info.py
+++ b/artifact_info.py
@@ -67,6 +67,10 @@
 # Tarball containing debug symbols for the given build.
 SYMBOLS = 'symbols'
 
+# A compressed tarball containing only sym files of debug symbols for the
+# given build.
+SYMBOLS_ONLY = 'symbols_only'
+
 # The factory test image.
 FACTORY_IMAGE = 'factory_image'