style: Record on a computed::Context whether we are computing a SMIL animated value.

components/style/gecko/media_queries.rs

@@ -642,6 +642,7 @@ impl Expression {
             in_media_query: true,
             // TODO: pass the correct value here.
             quirks_mode: quirks_mode,
+            for_smil_animation: false,
         };
 
         let required_value = match self.value {