Add sanity check for decreasing RTP timestamp in RtpToNtpMs.

webrtc/system_wrappers/source/rtp_to_ntp.cc

Index: webrtc/system_wrappers/source/rtp_to_ntp.cc
diff --git a/webrtc/system_wrappers/source/rtp_to_ntp.cc b/webrtc/system_wrappers/source/rtp_to_ntp.cc
index 6504737fd54bb32d84125bba2f8e4159a7005006..05091461d4123ff9bfbea041bd18187a0c411e02 100644
--- a/webrtc/system_wrappers/source/rtp_to_ntp.cc
+++ b/webrtc/system_wrappers/source/rtp_to_ntp.cc
@@ -110,6 +110,9 @@ bool RtpToNtpMs(int64_t rtp_timestamp,
                                &rtcp_timestamp_new)) {
     return false;
   }
+  if (rtcp_timestamp_new < rtcp_timestamp_old)
+    return false;
+
   double freq_khz;
   if (!CalculateFrequency(rtcp_ntp_ms_new,
                           rtcp_timestamp_new,