Replace the remaining scoped_ptr with unique_ptr in webrtc/modules/

(This is a re-land of https://codereview.webrtc.org/1921233002, which
got reverted for breaking Chromium.)

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1923133002

Cr-Commit-Position: refs/heads/master@{#12522}

webrtc/modules/rtp_rtcp/source/rtcp_receiver_help.h

@@ -90,7 +90,7 @@ public:
     bool xr_dlrr_item;
     std::unique_ptr<RTCPVoIPMetric>  VoIPMetric;
 
-    rtc::scoped_ptr<rtcp::TransportFeedback> transport_feedback_;
+    std::unique_ptr<rtcp::TransportFeedback> transport_feedback_;
 
 private:
     RTC_DISALLOW_COPY_AND_ASSIGN(RTCPPacketInformation);