VS2013 isos are mirrored to gs://chrome-wintoolchain/ (which is googler-
only accessible). This is used to isolate bots from external dependency
as detected by CHROME_HEADLESS=1 in the environment.

TBR=iannucci@chromium.org
R=hinoka@chromium.org
BUG=324987

Review URL: https://codereview.chromium.org/148613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@247398 0039d316-1c4b-4281-b951-d872f2087c98