Pass root ServerWindow id to FrameGenerator.

services/ui/ws/ids.h

Index: services/ui/ws/ids.h
diff --git a/services/ui/ws/ids.h b/services/ui/ws/ids.h
index a6268349313afe6001f54a0121f83b5a8c8bcccd..36a44b70375f01409a3def34baf9cd77c7a166ac 100644
--- a/services/ui/ws/ids.h
+++ b/services/ui/ws/ids.h
@@ -38,9 +38,9 @@ const ClientSpecificId kInvalidClientId = 0;
 // that embed roots use the client id in creating the window id to avoid
 // possible conflicts.
 struct WindowId {
-  WindowId(ClientSpecificId client_id, ClientSpecificId window_id)
+  constexpr WindowId(ClientSpecificId client_id, ClientSpecificId window_id)
       : client_id(client_id), window_id(window_id) {}
-  WindowId() : client_id(0), window_id(0) {}
+  constexpr WindowId() : client_id(0), window_id(0) {}
 
   bool operator==(const WindowId& other) const {
     return other.client_id == client_id && other.window_id == window_id;