Pass root ServerWindow id to FrameGenerator.

services/ui/ws/frame_generator.h

Index: services/ui/ws/frame_generator.h
diff --git a/services/ui/ws/frame_generator.h b/services/ui/ws/frame_generator.h
index d3cc1d8e0107f24004071a2109bd862af8564b1f..4ffbd043f75b33ac2cc867072ee2cc022a5b6fc0 100644
--- a/services/ui/ws/frame_generator.h
+++ b/services/ui/ws/frame_generator.h
@@ -14,6 +14,7 @@
 #include "cc/surfaces/local_frame_id.h"
 #include "cc/surfaces/surface_sequence.h"
 #include "cc/surfaces/surface_sequence_generator.h"
+#include "services/ui/ws/ids.h"
 #include "services/ui/ws/server_window_observer.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/native_widget_types.h"
@@ -49,6 +50,7 @@ class ServerWindowCompositorFrameSink;
 class FrameGenerator : public ServerWindowObserver {
  public:
   FrameGenerator(FrameGeneratorDelegate* delegate,
+                 WindowId root_window_id,
                  scoped_refptr<DisplayCompositor> display_compositor);
   ~FrameGenerator() override;