style: Add a temporary workaround for geckolib animations.

With needs_borrow we will be able to remove this, though we'll have to implement
clone_ and borrow_ methods for a bunch of properties.

components/style/animation.rs

@@ -422,6 +422,11 @@ pub fn maybe_start_animations<Impl: SelectorImplExt>(context: &SharedStyleContex
 {
     let mut had_animations = false;
 
+    // FIXME(emilio): Implement animations for geckolib.
+    if !new_style.is_servo() {
+        return false;
+    }
+
     let box_style = new_style.as_servo().get_box();
     for (i, name) in box_style.animation_name.0.iter().enumerate() {
         debug!("maybe_start_animations: name={}", name);

components/style/properties/properties.mako.rs

@@ -1259,6 +1259,7 @@ pub trait ComputedValues : Debug + Clone + Send + Sync + 'static {
         // yet - panics for non-Servo implementations.
         //
         // Used only for animations. Don't use it in other places.
+        fn is_servo(&self) -> bool;
         fn as_servo<'a>(&'a self) -> &'a ServoComputedValues;
         fn as_servo_mut<'a>(&'a mut self) -> &'a mut ServoComputedValues;
 
@@ -1310,7 +1311,11 @@ impl ComputedValues for ServoComputedValues {
         type Concrete${style_struct.trait_name} = style_structs::${style_struct.servo_struct_name};
     % endfor
 
+        #[inline]
+        fn is_servo(&self) -> bool { true }
+        #[inline]
         fn as_servo<'a>(&'a self) -> &'a ServoComputedValues { self }
+        #[inline]
         fn as_servo_mut<'a>(&'a mut self) -> &'a mut ServoComputedValues { self }
 
         fn new(custom_properties: Option<Arc<::custom_properties::ComputedValuesMap>>,

ports/geckolib/properties.mako.rs

@@ -74,6 +74,7 @@ impl ComputedValues for GeckoComputedValues {
 % endfor
 
     // These will go away, and we will never implement them.
+    fn is_servo(&self) -> bool { false }
     fn as_servo<'a>(&'a self) -> &'a ServoComputedValues { unimplemented!() }
     fn as_servo_mut<'a>(&'a mut self) -> &'a mut ServoComputedValues { unimplemented!() }