Modernize RtpRtcp factory function: use unique_ptr as return type

to clearly signal passed ownership.
Drop support for accepting nullptr clock to avoid copying the Configuration structure.
Update all calls in webrtc to the new factory function

Bug: None
Change-Id: Ic5a78da8e59ba3988a757a9d9634fa31499ce0db
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125901
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26994}

modules/rtp_rtcp/source/rtp_rtcp_impl.cc

@@ -41,6 +41,11 @@ constexpr int32_t kDefaultAudioReportInterval = 5000;
 
 RtpRtcp::Configuration::Configuration() = default;
 
+std::unique_ptr<RtpRtcp> RtpRtcp::Create(const Configuration& configuration) {
+  RTC_DCHECK(configuration.clock);
+  return absl::make_unique<ModuleRtpRtcpImpl>(configuration);
+}
+
 RtpRtcp* RtpRtcp::CreateRtpRtcp(const RtpRtcp::Configuration& configuration) {
   if (configuration.clock) {
     return new ModuleRtpRtcpImpl(configuration);