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_receiver.cc

@@ -323,6 +323,13 @@ int32_t VideoReceiver::Decode(uint16_t maxWaitTimeMs) {
     }
   }
 #endif
+
+  if (first_frame_received_()) {
+    LOG(LS_INFO) << "Received first "
+                 << (frame->Complete() ? "complete" : "incomplete")
+                 << " decodable video frame";
+  }
+
   const int32_t ret = Decode(*frame);
   _receiver.ReleaseFrame(frame);
   return ret;