Add ability to disable padding prioritization.

This allows trading off some potential media quality for CPU usage.

Bug: webrtc:8975
Change-Id: I447a03f596e9e711ba5d7038fe71f27bd80bf795
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/172085
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30936}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -149,6 +149,13 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
 
     bool need_rtp_packet_infos = false;
 
+    // If true, the RTP packet history will select RTX packets based on
+    // heuristics such as send time, retransmission count etc, in order to
+    // make padding potentially more useful.
+    // If false, the last packet will always be picked. This may reduce CPU
+    // overhead.
+    bool enable_rtx_padding_prioritization = false;
+
    private:
     RTC_DISALLOW_COPY_AND_ASSIGN(Configuration);
   };