Add sanity check for decreasing RTP timestamp in RtpToNtpMs.

webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc

Index: webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc
diff --git a/webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc b/webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc
index 78e4a52716dbd0c37fe6c76dbd228e5427982636..0cfd6896f72f9f29402c79eace322aa895789270 100644
--- a/webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc
+++ b/webrtc/system_wrappers/source/rtp_to_ntp_unittest.cc
@@ -136,6 +136,20 @@ TEST(WrapAroundTests, OldRtp_OldRtcpWrapped) {
   EXPECT_FALSE(RtpToNtpMs(timestamp, rtcp, &timestamp_in_ms));
 }
 
+TEST(RtpToNtpTests, FailsForDecreasingRtpTimestamp) {
+  const uint32_t kNtpSec1 = 3683354930;
+  const uint32_t kNtpFrac1 = 699925050;
+  const uint32_t kTimestamp1 = 2192705742;
+  const uint32_t kNtpSec2 = kNtpSec1;
+  const uint32_t kNtpFrac2 = kNtpFrac1 + kOneMsInNtpFrac;
+  const uint32_t kTimestamp2 = kTimestamp1 - kTimestampTicksPerMs;
+  RtcpList rtcp;
+  rtcp.push_front(RtcpMeasurement(kNtpSec1, kNtpFrac1, kTimestamp1));
+  rtcp.push_front(RtcpMeasurement(kNtpSec2, kNtpFrac2, kTimestamp2));
+  int64_t timestamp_in_ms = -1;
+  EXPECT_FALSE(RtpToNtpMs(kTimestamp1, rtcp, &timestamp_in_ms));
+}
+
 TEST(UpdateRtcpListTests, InjectRtcpSrWithEqualNtp) {
   RtcpList rtcp;
   uint32_t ntp_sec = 0;