Allow max 1 block per type in RTCP Extended Reports
Design of individual block in ExtendedReports packet suggest there is
no point to have more than one block per type.
This CL reduce complexity of having several blocks of the same type in
same report.
BUG=
webrtc:5260
Committed:
https://crrev.com/80ac24dd36b2df1c0faee5ae490b9a74db9c7464
Cr-Commit-Position: refs/heads/master@{#14855}