This removes places where the units types are implicitly left
uninitialized in network_types.h and adds rtc::Optional where needed.
Also removing the change indicator in the NetworkEstimate struct as it
is not used in practice.
Bug: webrtc:9155
Change-Id: I7e30e338effba96bd466ae91e380e6a8e90f66e1
Reviewed-on: https://webrtc-review.googlesource.com/73369
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23126}