Implement compute_distance for Angle.

When computing the distance between two filter functions, e.g.
HueRotate(0deg) and HueRotate(-360deg), we got an Err(()) and unwrap it
immediately in compute_filter_square_distance(), which causes a crash.
The root-cause is that we don't implement computed_distance of Angle, and we
assume we always get a valid result from compute_squared_distance() for
each filter function.

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

@@ -925,6 +925,13 @@ impl Animatable for Angle {
             }
         }
     }
+
+    #[inline]
+    fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
+        // Use the formula for calculating the distance between angles defined in SVG:
+        // https://www.w3.org/TR/SVG/animate.html#complexDistances
+        Ok((self.radians64() - other.radians64()).abs())
+    }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-percentage