Don't fallback to discrete animation within add_weighted() for Either<>.

For CSS Transitions we want this case to return Err() so we know that the two
values are not interpolable.

For CSS Animations/Web Animations we implement discrete animation as the
fallback behavior when Err() is returned.

components/style/properties/helpers/animated_properties.mako.rs

@@ -2410,10 +2410,7 @@ impl<T, U> Animatable for Either<T, U>
             (Either::Second(ref this), Either::Second(ref other)) => {
                 this.add_weighted(&other, self_portion, other_portion).map(Either::Second)
             },
-            _ => {
-                let result = if self_portion > other_portion {*self} else {*other};
-                Ok(result)
-            }
+            _ => Err(()),
         }
     }
 }