Let build/vs_toolchain.py run on non-Windows if requested.
Let build/vs_toolchain.py run on non-Windows if requested.

If `build/vs_toolchain.py update --force` is run once on non-Windows, the
script will start keeping a Windows toolchain in depot_tools up-to-date, and
it'll copy runtime dlls into the build directory, and so on.  (One can delete
build/win_toolchain.json to disable this behavior again.)

Depends on https://codereview.chromium.org/1284723006
BUG=495204

Review URL: https://codereview.chromium.org/1277913006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342917}

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