use gs.PUBLIC_BASE_HTTPS_URL in more places

Rather than copy & paste the URL, use the existing constant.

BUG=None
TEST=`buildbot/run_tests` works

Change-Id: If8f8bb1fbd1ea86c624b28c54f6b0c6fe1534b98
Reviewed-on: https://gerrit.chromium.org/gerrit/41452
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Ryan Cui <rcui@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 9cc0748..02e49f3 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -14,13 +14,14 @@
 from chromite.lib import cgroups
 from chromite.lib import commandline
 from chromite.lib import cros_build_lib
+from chromite.lib import gs
 from chromite.lib import locking
 from chromite.lib import osutils
 
 cros_build_lib.STRICT_SUDO = True
 
 
-DEFAULT_URL = 'https://commondatastorage.googleapis.com/chromiumos-sdk'
+DEFAULT_URL = gs.PUBLIC_BASE_HTTPS_URL + 'chromiumos-sdk'
 COMPRESSION_PREFERENCE = ('xz', 'bz2')
 
 # TODO(zbehan): Remove the dependency on these, reimplement them in python