Auto merge of #17477 - servo:closures-by-ref, r=emilio

Pass some 48-bytes closures by references instead of by value.

This reduced the generated code size by 74 KB: https://bugzilla.mozilla.org/show_bug.cgi?id=1375222#c4

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

@@ -582,7 +582,7 @@ impl AnimationValue {
                             &variables.url_data,
                             variables.from_shorthand,
                             &custom_props,
-                            |v| {
+                            &mut |v| {
                                 let declaration = match *v {
                                     DeclaredValue::Value(value) => {
                                         PropertyDeclaration::${prop.camel_case}(value.clone())