Optional: Use nullopt and implicit construction in /modules/rtp_rtcp

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

R=danilchap@webrtc.org

Bug: None
Change-Id: Ib4694d183f04d675f2ea66d39661fdffb2a984f1
Reviewed-on: https://webrtc-review.googlesource.com/23604
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20846}
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index 64b76db..d649fd6 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -267,7 +267,7 @@
 rtc::Optional<uint32_t> ModuleRtpRtcpImpl::FlexfecSsrc() const {
   if (rtp_sender_)
     return rtp_sender_->FlexfecSsrc();
-  return rtc::Optional<uint32_t>();
+  return rtc::nullopt;
 }
 
 void ModuleRtpRtcpImpl::IncomingRtcpPacket(const uint8_t* rtcp_packet,