Pass root ServerWindow id to FrameGenerator.

services/ui/ws/platform_display_init_params.h

 // Copyright 2016 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_PLATFORM_DISPLAY_INIT_PARAMS_H_
 #define SERVICES_UI_WS_PLATFORM_DISPLAY_INIT_PARAMS_H_
 
 #include <stdint.h>
 
 #include "base/memory/ref_counted.h"
 #include "services/ui/display/viewport_metrics.h"
 
 namespace ui {
 
 class DisplayCompositor;
-class GpuState;
 
 namespace ws {
 
+class ServerWindow;
+
 struct PlatformDisplayInitParams {
   PlatformDisplayInitParams();
+  PlatformDisplayInitParams(const PlatformDisplayInitParams& other);
   ~PlatformDisplayInitParams();
 
   scoped_refptr<DisplayCompositor> display_compositor;
 
   int64_t display_id;
+  ServerWindow* root_window = nullptr;
   display::ViewportMetrics metrics;
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_PLATFORM_DISPLAY_INIT_PARAMS_H_