Use NtpTime in RTCPSender::RtcpContext instead of pair of uint32_t
Use NtpTime in RTCPSender::RtcpContext instead of pair of uint32_t

BUG=None

Review-Url: https://codereview.webrtc.org/2577023002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15651}

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