Adds an Observer class to the DelegatedSourceListController so that we
can expose user-driven events from the delegated source list. This will
allow embedders to update their UI in response to changes to the state
from the delegated source list. Currently these events are: selection,
cancelled, and error.
Bug: chromium:1351576, chromium:1351577
Change-Id: I248bdb1c4410147ca1a0663eafda40b6b9345801
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272622
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#37986}
