Define accumulate method on Animatable trait

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

@@ -680,6 +680,15 @@ pub trait Animatable: Sized {
         self.add_weighted(other, 1.0, 1.0)
     }
 
+    /// [Accumulates][animation-accumulation] this value onto itself (|count| - 1) times then
+    /// accumulates |other| onto the result.
+    /// If |count| is zero, the result will be |other|.
+    ///
+    /// [animation-accumulation]: https://w3c.github.io/web-animations/#animation-accumulation
+    fn accumulate(&self, other: &Self, count: u64) -> Result<Self, ()> {
+        self.add_weighted(other, count as f64, 1.0)
+    }
+
     /// Compute distance between a value and another for a given property.
     fn compute_distance(&self, _other: &Self) -> Result<f64, ()>  { Err(()) }