xbuddy: Introduce the 'signed' image type

This change makes it possible to flash signed images from cros flash
using the `signed` alias.

BUG=chromium:852017
TEST=cros flash usb:// xbuddy://remote/eve-arcnext/latest-canary/signed
TEST=./xbuddy_unittest.py

Change-Id: I1c4c76ca0dd29845ef8639744745904e209a34eb
Reviewed-on: https://chromium-review.googlesource.com/1097609
Commit-Ready: Luis Hector Chavez <lhchavez@chromium.org>
Tested-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
diff --git a/downloader_unittest.py b/downloader_unittest.py
index 61bc880..0082f75 100755
--- a/downloader_unittest.py
+++ b/downloader_unittest.py
@@ -1,5 +1,5 @@
-#!/usr/bin/python2
-#
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
 # Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -67,7 +67,10 @@
     downloaded and the autotest tarball is attempted in the background.
     """
     self._SimpleDownloadOfTestSuites(
-        downloader.GoogleStorageDownloader(self._work_dir, self.archive_url))
+        downloader.GoogleStorageDownloader(
+            self._work_dir, self.archive_url,
+            downloader.GoogleStorageDownloader.GetBuildIdFromArchiveURL(
+                self.archive_url)))
 
   def testSimpleDownloadOfTestSuitesFromLocal(self):
     """Basic test_suites test.
@@ -98,7 +101,10 @@
   def testDownloadSymbolsFromGS(self):
     """Basic symbols download from Google Storage."""
     self._DownloadSymbolsHelper(
-        downloader.GoogleStorageDownloader(self._work_dir, self.archive_url))
+        downloader.GoogleStorageDownloader(
+            self._work_dir, self.archive_url,
+            downloader.GoogleStorageDownloader.GetBuildIdFromArchiveURL(
+                self.archive_url)))
 
   def testDownloadSymbolsFromLocal(self):
     """Basic symbols download from a Local Path."""