The Windows toolchain should be downloaded during gclient runhooks,
however because get_toolchain_if_necessary.py can either configure an
existing toolchain or download a toolchain there have been problems when
the toolchain is downloaded at an unexpected point. In particular, a
toolchain download during gn gen can lead to parallel downloads and
configuration failures.

The --nodownload option will be passed (after a subsequent change) by
build\vs_toolchain.py. A --nodownload option is needed instead of a more
positive --allowdownload option because of backwards compatibility. This
change will have no effect until the build\vs_toolchain.py change is
landed.

Bug: 584393, 662325
Change-Id: I9a0950e066744c0310e49e45d001a9113f1831cf
Reviewed-on: https://chromium-review.googlesource.com/c/1439805
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>