The Windows Metadata files are out of date

[JeffInChrist]

At the external folder, there are three *.winmd files added 3 years ago. They are out of date. For instance, the version of Windows.Foundation.UniversalApiContract is still 4.0 while the latest release is 10.0. So we cannot run unit tests against latest API's with these *.winmd files. I tried to test the signatures of several API's as below, getting an exception saying "System.Exception : Can't find property/type ..."

Windows.UI.Xaml.UIElement.ActualSize
Windows.UI.Xaml.UIElement.TransformMatrix
Windows.Foundation.GuidHelper
Windows.Foundation.GuidHelper.Equals

Shall we update them with the latest version?
https://github.com/mono/api-doc-tools/tree/develop/external/Windows/

[JeffInChrist]

@joelmartinez I tried to update them by myself but I could find only two of them at DevOps.
https://apidrop.visualstudio.com/binaries/_git/uwp-winrt

With the two in the apidrop, I can run the unit tests successfully and opened a draft PR.
#551

Is it fine to leave the other one (Windows.WinMD)? If so, I'll finalize the PR after #550 is merged.
cc: @anmeng10101