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/htmlimageelement.rs

@@ -47,6 +47,7 @@ struct ImageRequest {
     state: State,
     parsed_url: Option<Url>,
     source_url: Option<DOMString>,
+    #[ignore_heap_size_of = "Arc"]
     image: Option<Arc<Image>>,
     metadata: Option<ImageMetadata>,
 }