[PATCH 0/1] MR2559: widl: Add support for inherited runtime classes.

[Mohamad Al-Jaf (＠maljaf)]

Needed to build windows.ui.composition.idl.

Example code that needs this:

`
    [
        contract(Windows.Foundation.UniversalApiContract, 2.0),
        marshaling_behavior(agile),
        threading(both)
    ]
    runtimeclass ColorKeyFrameAnimation : Windows.UI.Composition.KeyFrameAnimation
    {
        [default] interface Windows.UI.Composition.IColorKeyFrameAnimation;
    }
`