add GS URL constructors

Yes, this code isn't that complicated, but I think the little bit of
magic that it performs in abstracting away buckets/constant names is
an improvement.

This also adds a simple shim over the SDK info as we want this URL in
the buildbot stages too, so move it to a common location.

BUG=chromium-os:36453
TEST=`buildbot/run_tests` worked
TEST=`cros_sdk --chroot=new` worked

Change-Id: I9450d2359240e1542d9f982c967e1a397590ef85
Reviewed-on: https://gerrit.chromium.org/gerrit/41539
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 02e49f3..ebeda08 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -14,14 +14,13 @@
 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
+from chromite.lib import toolchain
 
 cros_build_lib.STRICT_SUDO = True
 
 
-DEFAULT_URL = gs.PUBLIC_BASE_HTTPS_URL + 'chromiumos-sdk'
 COMPRESSION_PREFERENCE = ('xz', 'bz2')
 
 # TODO(zbehan): Remove the dependency on these, reimplement them in python
@@ -37,13 +36,13 @@
 def GetArchStageTarballs(version):
   """Returns the URL for a given arch/version"""
   extension = {'bz2':'tbz2', 'xz':'tar.xz'}
-  return ['%s/cros-sdk-%s.%s'
-          % (DEFAULT_URL, version, extension[compressor])
+  return [toolchain.GetSdkURL(suburl='cros-sdk-%s.%s'
+                              % (version, extension[compressor]))
           for compressor in COMPRESSION_PREFERENCE]
 
 
 def GetStage3Urls(version):
-  return ['%s/stage3-amd64-%s.tar.%s' % (DEFAULT_URL, version, ext)
+  return [toolchain.GetSdkURL(suburl='stage3-amd64-%s.tar.%s' % (version, ext))
           for ext in COMPRESSION_PREFERENCE]