c++ / mojo changes for 'external window mode'

services/ui/public/interfaces/window_tree_host.mojom

Index: services/ui/public/interfaces/window_tree_host.mojom
diff --git a/services/ui/public/interfaces/window_tree_host.mojom b/services/ui/public/interfaces/window_tree_host.mojom
index 8a349600a3d9b22aa5ca114ea6c949ad30e60315..8719433201bf2b9decf4604e96e90ae52e306bc5 100644
--- a/services/ui/public/interfaces/window_tree_host.mojom
+++ b/services/ui/public/interfaces/window_tree_host.mojom
@@ -17,9 +17,27 @@ interface WindowTreeHost {
   SetTitle(string title);
 };
 
+// WindowTreeHostFactoryRegistrar is the entry point to obtain a
+// WindowTreeHostFactory instance.
+// It also returns the mojo handle to the unique ws::WindowTree instance,
+// on the server-side.
+//
+// NOTE: WindowTreeHostFactoryRegistrar::Register and
+// WindowTreeHostFactory::CreateWindowTreeHost are put on separate interfaces,
+// so that the interface containing ::CreateWindowTreeHost is obtained by
+// calling ::Register. That eliminates the possibility of ::CreateWindowTreeHost
+// being called before ::Register.
+//
+interface WindowTreeHostFactoryRegistrar {
+   Register(WindowTreeHostFactory& window_tree_host_factory,
+            WindowTree& tree_request,
+            WindowTreeClient client);
+};
+
+// WindowTreeHostFactory triggers the creation of WindowTreeHost instances.
+// One WindowTree/WindowTreeClient pair can serve one or more WindowTreeHost
+// instances.
 interface WindowTreeHostFactory {
-  // Creates a new WindowTreeHost. |tree_client| is queried for the
-  // WindowManager.
-  CreateWindowTreeHost(WindowTreeHost& window_tree_host,
-                       WindowTreeClient tree_client);
+  // Creates a new WindowTreeHost.
+  CreateWindowTreeHost(WindowTreeHost& window_tree_host);
 };