buffet: Replace CallbackList with vector<Callback> We don't remove callback at runtime, so callbacks on WeakPtr are enough there. BUG=brillo:697 TEST=FEATURE=test emerge-gizmo buffet Change-Id: I02573979b208d0e9df62e59f4bb3bbec92608b71 Reviewed-on: https://chromium-review.googlesource.com/270279 Reviewed-by: Alex Vakulenko <[email protected]> Commit-Queue: Vitaly Buka <[email protected]> Tested-by: Vitaly Buka <[email protected]>