Restore depot_tools autoupdate in Windows.

update_depot_tools.bat

 @echo off
 :: 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.
 
 :: This batch file will try to sync the root directory.
 
 setlocal
 
 :: Windows freaks out if a file is overwritten while it's being executed.  Copy
@@ skipping 11 matching lines @@
 set GIT_URL=https://chromium.googlesource.com/chromium/tools/depot_tools.git
 
 :: Will download git, svn and python.
 call "%DEPOT_TOOLS_DIR%bootstrap\win\win_tools.bat"
 if errorlevel 1 goto :EOF
 :: Now clear errorlevel so it can be set by other programs later.
 set errorlevel=
 
 :: Initialize/update virtualenv.
 cd /d "%DEPOT_TOOLS_DIR%."
-python -u bootstrap\bootstrap.py --deps_file bootstrap\deps.pyl --quiet ENV
+call python.bat -u bootstrap\bootstrap.py --deps_file bootstrap\deps.pyl --quiet ENV
 if errorlevel 1 goto :EOF
 
 :: Shall skip automatic update?
 IF "%DEPOT_TOOLS_UPDATE%" == "0" GOTO :EOF
 
 :: We need either .\.svn\. or .\.git\. to be able to sync.
 IF EXIST "%DEPOT_TOOLS_DIR%.svn\." GOTO :SVN_UPDATE
 IF EXIST "%DEPOT_TOOLS_DIR%.git\." GOTO :GIT_UPDATE
 echo Error updating depot_tools, no revision tool found.
 goto :EOF
@@ skipping 25 matching lines @@
 call git fetch -q origin > NUL
 call git rebase -q origin/master > NUL
 if errorlevel 1 echo Failed to update depot_tools.
 goto :EOF
 
 
 :GIT_SVN_UPDATE
 cd /d "%DEPOT_TOOLS_DIR%."
 call git svn rebase -q -q
 goto :EOF