Add OAuth2 support for end users (i.e. 3-legged flow with the browser).

depot-tools-auth

Index: depot-tools-auth
diff --git a/fetch b/depot-tools-auth
similarity index 51%
copy from fetch
copy to depot-tools-auth
index bea6718c77ffc86eca095d4d9d0dbdb0a57bba5f..193afb97187ac1fb21faf32704cd97de30881d97 100755
--- a/fetch
+++ b/depot-tools-auth
@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
-# Copyright (c) 2013 The Chromium Authors. All rights reserved.
+# Copyright (c) 2015 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.
 
 base_dir=$(dirname "$0")
 
-PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/fetch.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/depot-tools-auth.py" "$@"