Pass root ServerWindow id to FrameGenerator.

services/ui/ws/platform_display_delegate.h

Index: services/ui/ws/platform_display_delegate.h
diff --git a/services/ui/ws/platform_display_delegate.h b/services/ui/ws/platform_display_delegate.h
index 721005818c20e3d0386d5843cc67797e5ba48d4f..0b81711e6c23a7d636a33f115d6006ea93b160aa 100644
--- a/services/ui/ws/platform_display_delegate.h
+++ b/services/ui/ws/platform_display_delegate.h
@@ -22,10 +22,6 @@ class ServerWindow;
 /// and responses to changes in viewport size.
 class PlatformDisplayDelegate {
  public:
-  // Creates the root window for this display. The new root window will have
-  // |size| in DIP initially.
-  virtual void CreateRootWindow(const gfx::Size& size) = 0;
-
   // Returns the root window of this display.
   virtual ServerWindow* GetRootWindow() = 0;