Revert 252633 "Switch to opt-out for toolchain"

> Switch to opt-out for toolchain
> 
> R=maruel@chromium.org
> BUG=323300
> 
> Review URL: https://codereview.chromium.org/174663005

TBR=scottmg@chromium.org

Review URL: https://codereview.chromium.org/175743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@252634 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools.bat

@@ -30,7 +30,8 @@ if errorlevel 1 goto :EOF
 :: Now clear errorlevel so it can be set by other programs later.
 set errorlevel=
 
-IF "%DEPOT_TOOLS_WIN_TOOLCHAIN%" == "0" GOTO :NOTOOLCHAIN
+:: 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=