WIP: Propagate SurfaceID up window tree hierarchy

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 7daf17da384388c6c2d3dfd05f2b95349ece3cfb..ecc551675d5e788f8f67ca180504a0e75d78e82d 100644
--- a/services/ui/ws/platform_display_delegate.h
+++ b/services/ui/ws/platform_display_delegate.h
@@ -23,6 +23,9 @@ struct ViewportMetrics;
 /// and responses to changes in viewport size.
 class PlatformDisplayDelegate {
  public:
+  // Returns the root window of the active user.
+  virtual ServerWindow* GetActiveRootWindow() = 0;
+
   // Returns the root window of this display.
   virtual ServerWindow* GetRootWindow() = 0;