Enable installing git by default on Windows.

bootstrap/win/win_tools.bat

 @echo off
 :: Copyright (c) 2011 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 script will try to find if svn and python are accessible and it not,
 :: it will try to download it and 'install' it in depot_tools.
 
 :: Sadly, we can't use SETLOCAL here otherwise it ERRORLEVEL is not correctly
 :: returned.
 
 set WIN_TOOLS_ROOT_URL=http://src.chromium.org/svn/trunk/tools
 
 :: Get absolute root directory (.js scripts don't handle relative paths well).
 pushd %~dp0..\..
 set WIN_TOOLS_ROOT_DIR=%CD%
 popd
 
 if "%1" == "force" (
   set WIN_TOOLS_FORCE=1
   shift /1
 )
 
 
 :GIT_CHECK
 :: If the batch file exists, skip the git check.
 if exist "%WIN_TOOLS_ROOT_DIR%\git.bat" goto :SVN_CHECK
-if "%GIT_TOOLS_FORCE%" == "1" goto :GIT_INSTALL
-:: The normal initialization will happen here when we're ready to deploy.
+if "%WIN_TOOLS_FORCE%" == "1" goto :GIT_INSTALL
+call git --version 2>nul 1>nul
+if errorlevel 1 goto :GIT_INSTALL
 goto :SVN_CHECK
 
 
 :GIT_INSTALL
 echo Installing git (avg 1-2 min download) ...
 :: git is not accessible; check it out and create 'proxy' files.
 if exist "%~dp0git.zip" del "%~dp0git.zip"
 cscript //nologo //e:jscript "%~dp0get_file.js" %WIN_TOOLS_ROOT_URL%/third_party/git_bin.zip "%~dp0git.zip"
 if errorlevel 1 goto :GIT_FAIL
 :: Cleanup svn directory if it was existing.
@@ skipping 95 matching lines @@
 goto :END
 
 
 :returncode
 set WIN_TOOLS_ROOT_URL=
 set WIN_TOOLS_ROOT_DIR=
 exit /b %ERRORLEVEL%
 
 :END
 call :returncode %ERRORLEVEL%