mus: Send display-id to client only for the root windows.

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index c08f52efce5cebc7d6926aab8221bb1da28e14ba..38908cd9c9bb86b5a54423c3d291e3e182b6c0ae 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -29,7 +29,6 @@ struct WindowData {
   // True if this window is visible. The window may not be drawn on screen (see
   // OnWindowParentDrawnStateChanged() for details).
   bool visible;
-  int64 display_id;
 };
 
 enum ErrorCode {
@@ -287,6 +286,7 @@ interface WindowTreeClient {
   OnEmbed(uint16 connection_id,
           WindowData root,
           WindowTree? tree,
+          int64 display_id,

[sky, 2016/05/27 16:25:56]

Document what display_id means here and below.

[sadrul, 2016/05/27 16:37:28]

Done.

           uint32 focused_window,
           bool parent_drawn);
 
@@ -306,7 +306,10 @@ interface WindowTreeClient {
   // Called in response to NewTopLevelWindow() successfully completing.
   // |parent_drawn| is true if the parent of the window is drawn, see
   // OnDrawnStateChanged() for details.
-  OnTopLevelCreated(uint32 change_id, WindowData data, bool parent_drawn);
+  OnTopLevelCreated(uint32 change_id,
+                    WindowData data,
+                    int64 display_id,
+                    bool parent_drawn);
 
   // Invoked when a window's bounds have changed.
   OnWindowBoundsChanged(uint32 window,