Don't clobber the default wrapping callback.

src/components/script/script_task.rs

@@ -616,10 +616,18 @@ impl ScriptTask {
             ptr.is_not_null()
         });
         unsafe {
-            JS_SetWrapObjectCallbacks((*js_runtime).ptr,
+            // JS_SetWrapObjectCallbacks clobbers the existing wrap callback,
+            // and JSCompartment::wrap crashes if that happens. The only way
+            // to retrieve the default callback is as the result of
+            // JS_SetWrapObjectCallbacks, which is why we call it twice.
+            let callback = JS_SetWrapObjectCallbacks((*js_runtime).ptr,
                                                      ptr::null(),
                                                      wrap_for_same_compartment,
                                                      ptr::null());
+            JS_SetWrapObjectCallbacks((*js_runtime).ptr,
+                                      callback,
+                                      wrap_for_same_compartment,
+                                      ptr::null());
         }
 
         let js_context = js_runtime.cx();