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 ad6b0464f48a3a02c4a2f627c8077f88ce2fde56..1664dca3254fc060f6e0f7a65354c494fd9e6fde 100644
--- a/mojo/services/navigation/navigation.mojom
+++ b/mojo/services/navigation/navigation.mojom
@@ -14,6 +14,11 @@ struct NavigationDetails {
   // TODO(aa): method, data, etc.
 };
 
+// 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.
 interface Navigator {