Demonstrate external-window-mode in mus-demo

services/ui/display/platform_screen.h

Index: services/ui/display/platform_screen.h
diff --git a/services/ui/display/platform_screen.h b/services/ui/display/platform_screen.h
index c266a57ee95adfc706c3baadb29653225479ad9a..776d723dff0d0e8cfd1568c6b47170f057405b7e 100644
--- a/services/ui/display/platform_screen.h
+++ b/services/ui/display/platform_screen.h
@@ -32,10 +32,12 @@ class PlatformScreen {
 
   // Triggers initial display configuration to start. On device this will
   // configuration the connected displays. Off device this will create one or
-  // more fake displays and pretend to configure them. A non-null |delegate|
-  // must be provided that will receive notifications when displays are added,
-  // removed or modified.
-  virtual void Init(PlatformScreenDelegate* delegate) = 0;
+  // more fake displays and pretend to configure them. In external window mode,
+  // no initial display will be created.  A non-null |delegate| must be provided
+  // that will receive notifications when displays are added, removed or
+  // modified.
+  virtual void Init(PlatformScreenDelegate* delegate,
+                    bool external_window_mode) = 0;
 
   // Handle requests from the platform to close a display.
   virtual void RequestCloseDisplay(int64_t display_id) = 0;