It's unclear whether or not we actually need this call to Measure. While adding border support to UILabel, I found that it can lead to layout cycles if our root Grid passes a different size value into Measure than it does into arrange.

In order to replicate how text is vertically aligned on the reference platform, we do the following in our ArrangeOverride:

Windows::Foundation::Size Label::ArrangeOverride(Windows::Foundation::Size finalSize) {
    // Make sure we render vertically-centered text if possible, else cap at the containing layer's height.
    TextBlock->Measure(finalSize);
    if (TextBlock->DesiredSize.Height >= finalSize.Height) {
        TextBlock->VerticalAlignment = Windows::UI::Xaml::VerticalAlignment::Top;
    } else {
        TextBlock->VerticalAlignment = Windows::UI::Xaml::VerticalAlignment::Center;
    }

    return __super::ArrangeOverride(finalSize);
}

However, the Xaml team says we probably don't need the extra call to TextBlock->Measure. If we can get rid of the call, we should, but doing so will require quite a bit of scrutiny and regression testing.