Make script build without impl<T: HeapSizeOf> HeapSizeOf for Arc<T>.

The removal of this impl is not included in this commit.
CC https://github.com/servo/heapsize/issues/37#issuecomment-249861171

components/script/dom/htmlobjectelement.rs

@@ -24,6 +24,7 @@ use string_cache::Atom;
 #[dom_struct]
 pub struct HTMLObjectElement {
     htmlelement: HTMLElement,
+    #[ignore_heap_size_of = "Arc"]
     image: DOMRefCell<Option<Arc<Image>>>,
 }