Frame marking RTP header extension (PART 1: implement extension)

Bug: webrtc:7765
Change-Id: I23896d121afd6be4bce5ff4deaf736149efebcdb
Reviewed-on: https://webrtc-review.googlesource.com/85200
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24695}

modules/rtp_rtcp/source/rtp_rtcp_impl.cc

@@ -50,6 +50,8 @@ RTPExtensionType StringToRtpExtensionType(const std::string& extension) {
     return kRtpExtensionVideoContentType;
   if (extension == RtpExtension::kVideoTimingUri)
     return kRtpExtensionVideoTiming;
+  if (extension == RtpExtension::kFrameMarkingUri)
+    return kRtpExtensionFrameMarking;
   if (extension == RtpExtension::kMidUri)
     return kRtpExtensionMid;
   RTC_NOTREACHED() << "Looking up unsupported RTP extension.";