Drop VP8 frames in case of duplicates in RtpFrameReferenceFinder.

BUG=webrtc:5514

Review-Url: https://codereview.webrtc.org/2734453002
Cr-Commit-Position: refs/heads/master@{#17090}
diff --git a/webrtc/modules/video_coding/rtp_frame_reference_finder.cc b/webrtc/modules/video_coding/rtp_frame_reference_finder.cc
index e49a81d..5926c52 100644
--- a/webrtc/modules/video_coding/rtp_frame_reference_finder.cc
+++ b/webrtc/modules/video_coding/rtp_frame_reference_finder.cc
@@ -334,8 +334,15 @@
       return;
     }
 
-    RTC_DCHECK((AheadOf<uint16_t, kPicIdLength>(frame->picture_id,
-                                               layer_info_it->second[layer])));
+    if (!(AheadOf<uint16_t, kPicIdLength>(frame->picture_id,
+                                          layer_info_it->second[layer]))) {
+      LOG(LS_WARNING) << "Frame with picture id " << frame->picture_id
+                      << " and packet range [" << frame->first_seq_num() << ", "
+                      << frame->last_seq_num() << "] already received, "
+                      << " dropping frame.";
+      return;
+    }
+
     ++frame->num_references;
     frame->references[layer] = layer_info_it->second[layer];
   }