message: Center-align the shield with the other tab row icons
  I started from here:
  but adding a padding of 3 was still off by one pixel, so it's 4 now.
  _enhance.png_
  ![image](https://user-images.githubusercontent.com/18356694/136219225-3fcffd48-79b4-4efc-a4c3-4b59f9878962.png)
  * [x] closes
diff: <mux:TabView.TabStripHeader>
  <!-- EA18 is the "Shield" glyph -->
  <FontIcon x:Uid="ElevationShield"
  - Margin="9,4,0,0"
  + Margin="9,4,0,4"
  FontFamily="Segoe MDL2 Assets"
  FontSize="16"
  Foreground="{ThemeResource SystemControlForegroundBaseMediumBrush}"
  