Adds ability for WindowManager to create display roots

services/ui/ws/window_manager_window_tree_factory.h

Index: services/ui/ws/window_manager_window_tree_factory.h
diff --git a/services/ui/ws/window_manager_window_tree_factory.h b/services/ui/ws/window_manager_window_tree_factory.h
index df5c4b0e8c59525a162f4ff7a931a7e23dab4f8d..ce56479db8b760ec704d0512edfa91f6048c7af8 100644
--- a/services/ui/ws/window_manager_window_tree_factory.h
+++ b/services/ui/ws/window_manager_window_tree_factory.h
@@ -34,7 +34,8 @@ class WindowManagerWindowTreeFactory
 
   // mojom::WindowManagerWindowTreeFactory:
   void CreateWindowTree(mojom::WindowTreeRequest window_tree_request,
-                        mojom::WindowTreeClientPtr window_tree_client) override;
+                        mojom::WindowTreeClientPtr window_tree_client,
+                        bool window_manager_creates_roots) override;
 
  private:
   // Used by tests.
@@ -50,7 +51,7 @@ class WindowManagerWindowTreeFactory
   mojo::Binding<mojom::WindowManagerWindowTreeFactory> binding_;
 
   // Owned by WindowServer.
-  WindowTree* window_tree_;
+  WindowTree* window_tree_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(WindowManagerWindowTreeFactory);
 };