Win32: GridLength doesn't match UWP GridLength

Describe the bug
Microsoft.UI.Xaml.GridLength is very different on Win32 vs UWP.

For instance Microsoft.UI.Xaml.GridLength has different constructors between UWP and .NET Core, and the .NET Core one is immutable. So I had to do this:

#if NETFX_CORE
        var size = new Microsoft.UI.Xaml.GridLength() { Value = 1, GridUnitType = Microsoft.UI.Xaml.GridUnitType.Star };
#elif NETCOREAPP
        var size = new Microsoft.UI.Xaml.GridLength(1, Microsoft.UI.Xaml.GridUnitType.Star);
#endif

Win32 version:

    public struct GridLength
    {
        public GridLength(double pixels);
        public GridLength(double value, GridUnitType type);

        public static GridLength Auto { get; }
        public double Value { get; }
        public GridUnitType GridUnitType { get; }
        public bool IsAbsolute { get; }
        public bool IsAuto { get; }
        public bool IsStar { get; }

        public override bool Equals(object oCompare);
        public bool Equals(GridLength gridLength);
        public override int GetHashCode();
        public override string ToString();

        public static bool operator ==(GridLength gl1, GridLength gl2);
        public static bool operator !=(GridLength gl1, GridLength gl2);
    }

UWP version:

    public struct GridLength
    {
        public double Value;
        public GridUnitType GridUnitType;
    }

Steps to reproduce the bug
Inspect the Microsoft.UI.Xaml.GridLength type on both Win32 and UWP and notice the big differences.

Expected behavior
Win32 looks like UWP version.

Version Info
3.0.0-preview1.200515.3

NuGet package version: 3.0.0-preview1.200515.3

Additional context

Start by locating the Microsoft.UI.Xaml.GridLength projection and compare its Win32 API surface with the UWP definition shown in the issue. Reproduce the constructor and mutability differences, then verify that the Win32 type exposes the expected UWP-compatible fields and behavior.