Add logging of decoded video frames.

This CL adds the possibility to log metainformation about
decoded frames in RTC event log, including encoding parsing
and tests. It will be wired up in a followup CL.


Bug: webrtc:8802
Change-Id: Ied598b266513d0f63fce0484d741af1782607e74
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181061
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31873}

api/rtc_event_log/rtc_event.h

@@ -52,7 +52,8 @@ class RtcEvent {
     VideoSendStreamConfig,
     GenericPacketSent,
     GenericPacketReceived,
-    GenericAckReceived
+    GenericAckReceived,
+    FrameDecoded
   };
 
   RtcEvent();