Don't forget to return self

To retain the functionality where we can chain
function calls to add, link we need to return the
object we are acting on (instead of none).

Change-Id: Iadf0f9d3a21b6d079c1edd29063196180f5222ed
Fix another instance of callback comparison

Seems like I missed another instance of
using the new 'is_same_callback' method instead
of comparing callbacks directly via 'is'

Partial-Bug: #1257550

Change-Id: I7be2647c9be2c97f6786637d3fce53640e31a3b8