Move toolchain update control into src, but keep download logic in depot_tools

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 12 matching lines @@
 
 :: Will download svn and python.
 :: If you don't want to install the depot_tools version of these tools, remove
 :: the 'force' option on the next command. The tools will be installed only if
 :: not already in the PATH environment variable.
 call "%DEPOT_TOOLS_DIR%bootstrap\win\win_tools.bat" force
 if errorlevel 1 goto :EOF
 :: Now clear errorlevel so it can be set by other programs later.
 set errorlevel=
 
-:: Opt-in for now.
-IF "%DEPOT_TOOLS_WIN_TOOLCHAIN%" == "" GOTO :NOTOOLCHAIN
-call python %~dp0win_toolchain\get_toolchain_if_necessary.py
-if errorlevel 1 goto :EOF
-set errorlevel=
-:NOTOOLCHAIN
-
 :: 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 17 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
+goto :EOF