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}
diff --git a/webrtc/modules/video_coding/jitter_buffer.cc b/webrtc/modules/video_coding/jitter_buffer.cc
index f048b0a..4e012f6 100644
--- a/webrtc/modules/video_coding/jitter_buffer.cc
+++ b/webrtc/modules/video_coding/jitter_buffer.cc
@@ -1282,9 +1282,13 @@
   if (frame.IsSessionComplete()) {
     if (frame.FrameType() == kVideoFrameKey) {
       ++receive_statistics_.key_frames;
+      if (receive_statistics_.key_frames == 1) {
+        LOG(LS_INFO) << "Received first complete key frame";
+      }
     } else {
       ++receive_statistics_.delta_frames;
     }
+
     if (stats_callback_ != NULL)
       stats_callback_->OnFrameCountsUpdated(receive_statistics_);
   }