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/workerglobalscope.rs

@@ -83,6 +83,7 @@ pub struct WorkerGlobalScope {
     worker_id: WorkerId,
     pipeline_id: PipelineId,
     worker_url: Url,
+    #[ignore_heap_size_of = "Arc"]
     closing: Option<Arc<AtomicBool>>,
     #[ignore_heap_size_of = "Defined in js"]
     runtime: Runtime,