Pass root ServerWindow id to FrameGenerator.

services/ui/ws/ids.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SERVICES_UI_WS_IDS_H_
 #define SERVICES_UI_WS_IDS_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 20 matching lines @@
 //
 // This model works when the client initiates creation of windows, which is
 // the typical use case. Embed roots and the WindowManager are special, they
 // get access to windows created by other clients. These clients see the
 // id assigned on the server. Such clients have to take care that they only
 // create windows using their client id. To do otherwise could result in
 // multiple windows having the same ClientWindowId. WindowTree enforces
 // 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;
   }
 
   bool operator!=(const WindowId& other) const { return !(*this == other); }
 
   bool operator<(const WindowId& other) const {
     return std::tie(client_id, window_id) <
            std::tie(other.client_id, other.window_id);
@@ skipping 56 matching lines @@
 template <>
 struct hash<ui::ws::WindowId> {
   size_t operator()(const ui::ws::WindowId& id) const {
     return hash<ui::Id>()(WindowIdToTransportId(id));
   }
 };
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // SERVICES_UI_WS_IDS_H_