This is a reland of 68ac4e43

Original change's description:
> Add support for building in Flutter Windows UWP configuration
>
> Bug: angleproject:5527
> Change-Id: Idf5a4cbb6f84b24fa2448157cab1b6a3bce4d8be
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2620580
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>

Bug: angleproject:5527
Change-Id: I3d5c3a35d73e3946ae40c779e76e0343039bf660
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2693639Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>