Add support for downloading Windows VS toolchain from a HTTP URL.
This is useful for developers that do not have access to the
Chrome toolchain.

If the developer specifies DEPOT_TOOLS_WIN_TOOLCHAIN_HTTP_BASE_URL
environment variable, the toolchain will be downloaded from that
url.

Bug: 830569
Change-Id: I41d815a4460085c3b028f56f2e01adc68d8410fe
Reviewed-on: https://chromium-review.googlesource.com/1002892
Commit-Queue: Kimmo Kinnunen FI <kkinnunen@nvidia.com>
Reviewed-by: Scott Graham <scottmg@chromium.org>