Adds ability for WindowManager to create display roots

services/ui/public/interfaces/window_manager.mojom

Index: services/ui/public/interfaces/window_manager.mojom
diff --git a/services/ui/public/interfaces/window_manager.mojom b/services/ui/public/interfaces/window_manager.mojom
index a2a5e50b075b0a0ed092212815253b67fb027f1a..aa340ba650b857fffb8ac9f6ec7654e0ea8808d6 100644
--- a/services/ui/public/interfaces/window_manager.mojom
+++ b/services/ui/public/interfaces/window_manager.mojom
@@ -270,6 +270,12 @@ interface WindowManagerClient {
   AddAccelerators(array<WmAccelerator> accelerators) => (bool success);
   RemoveAccelerator(uint32 id);
 
+  // Sets the root of a particular display. This is only applicable when the
+  // WindowTree was created with a value of false for
+  // |automatically_create_display_roots| (see
+  // WindowManagerWindowTreeFactory::CreateWindowTree() for details).
+  SetDisplayRoot(int64 display_id, uint32 window_id) => (bool success);
+
   // The window manager has completed a request with the specific change id.
   WmResponse(uint32 change_id, bool response);