[NaCl SDK] Refactor sdk_update*.

native_client_sdk/src/build_tools/sdk_tools/command/sources.py

+# Copyright (c) 2012 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import download
+from sdk_update_common import Error
+
+def AddSource(config, url):
+  try:
+    download.UrlOpen(url)
+  except Exception as e:
+    raise Error('Not adding %s, unable to load URL.\n  %s' % (url, e))
+  config.AddSource(url)
+
+
+def RemoveSource(config, url):
+  if url == 'all':
+    config.RemoveAllSources()
+  else:
+    config.RemoveSource(url)
+
+
+def ListSources(config):
+  if config.sources:
+    print 'Installed sources:'
+    for s in config.sources:
+      print '  ' + s
+  else:
+    print 'No external sources installed.'