The moment we've all been waiting for. Step 1: Add type alias (this CL). Step 2: Migrate all uses of RTCStatsMember<T> to absl::optional<T>. Step 3: Delete type alias. Bug: webrtc:15164 Change-Id: I00a7202c0b684fb2c57fcad4f501bccc167f1fa3 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/334680 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@{#41593}