Switch bot_update to call gclient.bat on windows
Switch bot_update to call gclient.bat on windows

BUG=339171

Review URL: https://codereview.chromium.org/164093003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@251103 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build