Pass device scale factor from display to ws.

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..f0dfa13a55baf76d33a39b52905194bd9b4d5f74 100644
--- a/services/ui/ws/platform_display_delegate.h
+++ b/services/ui/ws/platform_display_delegate.h
@@ -5,14 +5,14 @@
 #ifndef SERVICES_UI_WS_PLATFORM_DISPLAY_DELEGATE_H_
 #define SERVICES_UI_WS_PLATFORM_DISPLAY_DELEGATE_H_
 
-#include "services/ui/ws/ids.h"
-
-namespace ui {
-class Event;
+namespace gfx {
+class Size;
 }
 
 namespace ui {
 
+class Event;
+
 namespace ws {
 
 class ServerWindow;
@@ -23,6 +23,10 @@ struct ViewportMetrics;
 /// 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;