Pass root ServerWindow id to FrameGenerator.

services/ui/ws/display.h

Index: services/ui/ws/display.h
diff --git a/services/ui/ws/display.h b/services/ui/ws/display.h
index 98edd42f3bd8b2daba72110722755867e41eb32f..c5c74c005f12d3cfa8888fbec81415f5ef55c7c8 100644
--- a/services/ui/ws/display.h
+++ b/services/ui/ws/display.h
@@ -163,8 +163,11 @@ class Display : public PlatformDisplayDelegate,
   void CreateWindowManagerDisplayRootFromFactory(
       WindowManagerWindowTreeFactory* factory);
 
+  // Creates the root ServerWindow for this display. The root window will have
+  // |size| in DIP.
+  void CreateRootWindow(const gfx::Size& size);
+
   // PlatformDisplayDelegate:
-  void CreateRootWindow(const gfx::Size& size) override;
   ServerWindow* GetRootWindow() override;
   bool IsInHighContrastMode() override;
   void OnEvent(const ui::Event& event) override;