Per comments in linked bug, this is simply moved from chromium. The
only new thing is hooking it into update_depot_tools.bat, and
updating a few paths in the scripts.

This is currently opt-in via "set DEPOT_TOOLS_WIN_TOOLCHAIN=1" but
that will be removed once it's ready for deployment.

R=iannucci@chromium.org, maruel@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/135933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@244983 0039d316-1c4b-4281-b951-d872f2087c98