Allow empty keyframe and keyframes with non-animatable properties.

We need to create CSS animations that have empty keyframe or keyframes
which have only invalid properties or non-animatable properties to fire
animation events for such animations.

tests/unit/style/lib.rs

@@ -25,6 +25,7 @@ extern crate test;
 mod animated_properties;
 mod attr;
 mod cache;
+mod keyframes;
 mod logical_geometry;
 mod media_queries;
 mod owning_handle;