Enable uwp feature when building with --uwp.

python/servo/command_base.py

@@ -846,6 +846,7 @@ install them, let us know by filing a bug!")
         if uwp:
             features.append("canvas2d-raqote")
             features.append("no_wgl")
+            features.append("uwp")
         else:
             # Non-UWP builds provide their own libEGL via mozangle.
             features.append("egl")