rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../../build/webrtc.gni")
 
@@ skipping 123 matching lines @@
     "source/rtp_sender.cc",
     "source/rtp_sender.h",
     "source/rtp_sender_audio.cc",
     "source/rtp_sender_audio.h",
     "source/rtp_sender_video.cc",
     "source/rtp_sender_video.h",
     "source/rtp_utility.cc",
     "source/rtp_utility.h",
     "source/ssrc_database.cc",
     "source/ssrc_database.h",
+    "source/time_util.cc",
+    "source/time_util.h",
     "source/tmmbr_help.cc",
     "source/tmmbr_help.h",
     "source/video_codec_information.h",
     "source/vp8_partition_aggregator.cc",
     "source/vp8_partition_aggregator.h",
   ]
 
   configs += [ "../..:common_config" ]
   public_configs = [ "../..:common_inherited_config" ]
 
@@ skipping 12 matching lines @@
   if (is_win) {
     cflags = [
       # TODO(jschuh): Bug 1348: fix this warning.
       "/wd4267",  # size_t to int truncations
 
       # TODO(kjellander): Bug 261: fix this warning.
       "/wd4373",  # virtual function override.
     ]
   }
 }