Added log messages for some important call setup events:

- First audio RTP packet sent / received
 - First RTP packet of the first video frame sent / received
 - Last RTP packet of the first video frame sent / received
These timestamps should make it easier to measure how fast the call
becomes established from the user's perspective.

Review URL: https://codereview.webrtc.org/1765443002

Cr-Commit-Position: refs/heads/master@{#12287}

webrtc/modules/video_coding/video_coding_impl.h

@@ -16,6 +16,7 @@
 #include <memory>
 #include <vector>
 
+#include "webrtc/base/onetimeevent.h"
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/base/thread_checker.h"
 #include "webrtc/modules/video_coding/codec_database.h"
@@ -219,6 +220,7 @@ class VideoReceiver {
   VCMProcessTimer _retransmissionTimer;
   VCMProcessTimer _keyRequestTimer;
   QpParser qp_parser_;
+  ThreadUnsafeOneTimeEvent first_frame_received_;
 };
 
 }  // namespace vcm