Replace uses of for foo in bar.iter() and for foo in bar.iter_mut()

closes #7197

components/style/viewport.rs

@@ -363,7 +363,7 @@ impl ViewportConstraints {
         let mut orientation = Orientation::Auto;
 
         // collapse the list of declarations into descriptor values
-        for declaration in rule.declarations.iter() {
+        for declaration in &rule.declarations {
             match declaration.descriptor {
                 ViewportDescriptor::MinWidth(value) => min_width = Some(value),
                 ViewportDescriptor::MaxWidth(value) => max_width = Some(value),