Renames DEPOT_TOOLS_WIN_TOOLCHAIN_HTTP_BASE_URL to
DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_LOCATION. This environment variable can
now point to a local directory where the Windows SDK zip file is stored.
This allows non-Googlers to cross-compile Chromium for Windows.

Bug: 852347
Change-Id: I00650e84247f35b4b8cfba204e0f2afd0882b69b
Reviewed-on: https://chromium-review.googlesource.com/1098256
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>