make dom_struct derive HeapSizeOf,

closes #7357

components/script/dom/htmloutputelement.rs

@@ -15,7 +15,6 @@ use dom::validitystate::ValidityState;
 use util::str::DOMString;
 
 #[dom_struct]
-#[derive(HeapSizeOf)]
 pub struct HTMLOutputElement {
     htmlelement: HTMLElement
 }
@@ -54,4 +53,3 @@ impl<'a> HTMLOutputElementMethods for &'a HTMLOutputElement {
         ValidityState::new(window.r())
     }
 }
-