Split surface id and simplify connecting to surfaces service

mojo/services/surfaces/public/interfaces/surface_id.mojom

Index: mojo/services/surfaces/public/interfaces/surface_id.mojom
diff --git a/mojo/services/surfaces/public/interfaces/surface_id.mojom b/mojo/services/surfaces/public/interfaces/surface_id.mojom
index eb40242c50566ee607ce1893e34ea399ef0b8b3e..d68e2e7b69b65381e39e9c37398fb0cb69152da6 100644
--- a/mojo/services/surfaces/public/interfaces/surface_id.mojom
+++ b/mojo/services/surfaces/public/interfaces/surface_id.mojom
@@ -4,6 +4,11 @@
 
 module mojo;
 
+// A surface ID is composed of two parts, a local part and a namespace. The
+// local part is allocated by the client using any scheme that avoids
+// duplicates. The namespace is allocated by the service and will be different
+// for each client (aka each connection to mojo.Surface).
 struct SurfaceId {
-  uint64 id;
+  uint32 local;
+  uint32 id_namespace;
 };