Turn 'browser' app into a 'launcher_ui' app that window_manager embeds.

mojo/services/navigation/navigation.mojom

Index: mojo/services/navigation/navigation.mojom
diff --git a/mojo/services/navigation/navigation.mojom b/mojo/services/navigation/navigation.mojom
index 34c82a6b31945c3e8f1683d57bb8ed277e13dcb7..82fa3de6dd34e07735efc895254b1cb0e4510ba3 100644
--- a/mojo/services/navigation/navigation.mojom
+++ b/mojo/services/navigation/navigation.mojom
@@ -18,6 +18,11 @@ struct ResponseDetails {
   handle<data_pipe_consumer> response_body_stream;
 };
 
+// Embedders that support navigation of implement this interface.
+interface NavigatorHost {
+  RequestNavigate(uint32 source_node_id, NavigationDetails details);
+};
+
 // Applications implement this interface to support navigation of their views
 // by embedders.
 // |response_details| can be NULL when a navigation was not the result of a