One in series of CLs to migrate from Members() to Attributes() to
unblock code deletion.
Adds and makes use of has_value(), holds_alternative<T>() and get<T>()
convenience methods.
Bug: webrtc:15164
Change-Id: I2ae9869a5d1de3f3875c70494c9781d69da7f936
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/334160
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/main@{#41510}
bfee961786