Make the impl of JSTraceable for LayoutRPC sound with a Send bound

components/script/dom/bindings/trace.rs

@@ -391,7 +391,8 @@ unsafe impl<T> JSTraceable for IpcSender<T> where T: Deserialize + Serialize {
     }
 }
 
-unsafe impl JSTraceable for Box<LayoutRPC + 'static> {
+// Safe thanks to the Send bound.
+unsafe impl JSTraceable for Box<LayoutRPC + Send + 'static> {
     #[inline]
     unsafe fn trace(&self, _: *mut JSTracer) {
         // Do nothing

components/script/dom/window.rs

@@ -196,7 +196,7 @@ pub struct Window {
 
     /// A handle to perform RPC calls into the layout, quickly.
     #[ignore_heap_size_of = "trait objects are hard"]
-    layout_rpc: Box<LayoutRPC + 'static>,
+    layout_rpc: Box<LayoutRPC + Send + 'static>,
 
     /// The current size of the window, in pixels.
     window_size: Cell<Option<WindowSizeData>>,
@@ -1538,7 +1538,7 @@ impl Window {
                parent_info: Option<(PipelineId, FrameType)>,
                window_size: Option<WindowSizeData>)
                -> Root<Window> {
-        let layout_rpc: Box<LayoutRPC> = {
+        let layout_rpc: Box<LayoutRPC + Send> = {
             let (rpc_send, rpc_recv) = channel();
             layout_chan.send(Msg::GetRPC(rpc_send)).unwrap();
             rpc_recv.recv().unwrap()