style: Generate combined transform just once.

Differential Revision: https://phabricator.services.mozilla.com/D18567

components/style/properties/cascade.rs

@@ -629,6 +629,10 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
 
         #[cfg(feature = "gecko")]
         {
+            if let Some(display) = builder.get_box_if_mutated() {
+                display.generate_combined_transform();
+            }
+
             if let Some(bg) = builder.get_background_if_mutated() {
                 bg.fill_arrays();
             }