Adds ability for WindowManager to create display roots

services/ui/public/interfaces/window_manager_window_tree_factory.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 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);
 };