The current SDK on the Windows machines appears to be version 10.0.14393.0, based on manual looking around done by @aam. As of today, the latest version is 10.0.19041.0.

The Chromium team maintains a CIPD package including the SDK; however, it does not include the References/<version> subdirectory, which is a requirement for the cppwinrt.exe tool, which we use to generate the WinRT C++ header libraries from the SDK.

Chrome's packaging docs are available at go/chromium-msvc-toolchain and the Windows SDK package is generated via the package_from_installed.py script in the depot_tools repository.

I talked to brucedawson@ and he'd be fine with us adding a flag that allows the script to be run with a flag that controls whether or not the References/<version> directory is included in the image. We can then use that script to produce our own CIPD packages of the Windows SDK.