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>
2 files changed