Treat negative ntp time as unset.

webrtc/video/vie_encoder.cc

Index: webrtc/video/vie_encoder.cc
diff --git a/webrtc/video/vie_encoder.cc b/webrtc/video/vie_encoder.cc
index f78633d532f438f809dce7453fb57accfeff0dd7..adf8fe7eb4677a9e497b4292e8a7e244fe354b9f 100644
--- a/webrtc/video/vie_encoder.cc
+++ b/webrtc/video/vie_encoder.cc
@@ -464,7 +464,7 @@ void ViEEncoder::OnFrame(const VideoFrame& video_frame) {
 
   // Capture time may come from clock with an offset and drift from clock_.
   int64_t capture_ntp_time_ms;
-  if (video_frame.ntp_time_ms() != 0) {
+  if (video_frame.ntp_time_ms() > 0) {
     capture_ntp_time_ms = video_frame.ntp_time_ms();
   } else if (video_frame.render_time_ms() != 0) {
     capture_ntp_time_ms = video_frame.render_time_ms() + delta_ntp_internal_ms_;