In flutter/engine#25993, we disabled the Windows UWP trybot and builder shards.

During the resolution of #81837, we migrated from our old Fuchsia bot images, which included Windows SDK 10.0.18362.0, to Chromium bot images, which included the relatively ancient Windows SDK 10.0.14393.0. Unfortunately, this older SDK didn't contain all necessary APIs to build the UWP embedder. As a result, we disabled the shard given its current pre-alpha status and the fact that the tree had been closed for 3 days for unrelated reasons (due to the certificate issue noted above).

#81994 tracks the work to package Windows SDK 10.0.19041.0 (the most recent SDK as of May 2021) as a CIPD bundle and install it as part of gclient sync. Once that's done, build/win/generate_winrt_headers.py in the buildroot repo can be updated to build the WinRT headers from that SDK.