Use local ids for Surfaces APIs that can only apply to local surfaces

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 d68e2e7b69b65381e39e9c37398fb0cb69152da6..197e6f8bdc08c9efdbbae11e0335e93e30f7f9d9 100644
--- a/mojo/services/surfaces/public/interfaces/surface_id.mojom
+++ b/mojo/services/surfaces/public/interfaces/surface_id.mojom
@@ -8,6 +8,12 @@ module mojo;
 // 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).
+//
+// The special id_namespace value 0 is equivalent to the namespace of the
+// client. This can be used to create, destroy and submit frames to
+// surfaces before learning the namespace and to reference other surfaces
+// owned by the same client. The actual id namespace must be used to pass
+// surface ids to other clients for them to reference.
 struct SurfaceId {
   uint32 local;
   uint32 id_namespace;