Adds ability for WindowManager to create display roots

services/ui/public/interfaces/window_manager_window_tree_factory.mojom

Index: services/ui/public/interfaces/window_manager_window_tree_factory.mojom
diff --git a/services/ui/public/interfaces/window_manager_window_tree_factory.mojom b/services/ui/public/interfaces/window_manager_window_tree_factory.mojom
index 152b3263a784028e39a75ddc6a173db228666939..04ad1f3c4f9e68f3abecab7b19feb9c19f39b071 100644
--- a/services/ui/public/interfaces/window_manager_window_tree_factory.mojom
+++ b/services/ui/public/interfaces/window_manager_window_tree_factory.mojom
@@ -7,10 +7,17 @@ module ui.mojom;
 import "services/ui/public/interfaces/window_manager_constants.mojom";
 import "services/ui/public/interfaces/window_tree.mojom";
 
-// Interface used by the WindowManager to obtain a WindowTree. The
-// WindowManager is informed of the roots (one per display) by way of
-// WmNewDisplayAdded(). See it for details.
+// Interface used by the WindowManager to obtain a WindowTree. Server may be
+// configured in two distinct modes by way of the
+// |automatically_create_display_roots| parameter:
+// . If true, then mus creates windows for displays as displays are added and
+//   calls WmNewDisplayAdded() (see it for more details).
+// . If false, the client is responsible for creating and setting windows for
+//   displays (via SetDisplayRoot()). In this mode it is assumed the client is
+//   detecting displays via another mechanism.
 interface WindowManagerWindowTreeFactory {
   // NOTE: it is expected this is called only once.
-  CreateWindowTree(WindowTree& tree_request, WindowTreeClient client);
+  CreateWindowTree(WindowTree& tree_request,
+                   WindowTreeClient client,
+                   bool automatically_create_display_roots);
 };