Move the event attach/detach logic on unload from event.js to

extensions/renderer/object_backed_native_handler.h

Index: extensions/renderer/object_backed_native_handler.h
diff --git a/extensions/renderer/object_backed_native_handler.h b/extensions/renderer/object_backed_native_handler.h
index 7fe4b1fe305e9d230f4527ad671bf5a38169e7fa..843e13bf466ceaa867730b0b942e33509b0e5219 100644
--- a/extensions/renderer/object_backed_native_handler.h
+++ b/extensions/renderer/object_backed_native_handler.h
@@ -38,6 +38,10 @@ class ObjectBackedNativeHandler : public NativeHandler {
   // Installs a new 'route' from |name| to |handler_function|. This means that
   // NewInstance()s of this ObjectBackedNativeHandler will have a property
   // |name| which will be handled by |handler_function|.
+  //
+  // Routed functions are destroyed along with the destruction of this class,
+  // and are never called back into, therefore it's safe for |handler_function|
+  // to bind to base::Unretained.
   void RouteFunction(const std::string& name,
                      const HandlerFunction& handler_function);