Add timestamp to log message in generic_decoder.cc

Bug: None
Change-Id: Ib558247d887aff880853ef824f8d80d8e7e4feee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222610
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34319}

modules/video_coding/generic_decoder.cc

@@ -113,7 +113,8 @@ void VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage,
 
   if (!frameInfo) {
     RTC_LOG(LS_WARNING) << "Too many frames backed up in the decoder, dropping "
-                           "this one.";
+                           "frame with timestamp "
+                        << decodedImage.timestamp();
     return;
   }