libweave: use base::CallbackListSubscription in Mock and test

StateChangeQueueInterface::AddOnStateUpdatedCallback will return
base::CallbackListSubscription instead of
base::CallbackList<void(UpdateID)>::Subscription* after libchrome
r860220 uprev.
Change the Mock and expected behavior accordingly.

BUG=chromium:1179608
TEST=FEATURES=test emerge-gale libweave with old and new libchrome

Change-Id: I69f9f26be28a402953eab3fb8082b77eb4cc6136
Reviewed-on: https://chromium-review.googlesource.com/c/weave/libweave/+/2822582
Tested-by: Grace Cham <hscham@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Grace Cham <hscham@chromium.org>
2 files changed