rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/rtp_rtcp.gypi

 # Copyright (c) 2011 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.
 
 {
   'targets': [
@@ skipping 91 matching lines @@
         'source/rtp_header_extension.cc',
         'source/rtp_header_extension.h',
         'source/rtp_receiver_impl.cc',
         'source/rtp_receiver_impl.h',
         'source/rtp_sender.cc',
         'source/rtp_sender.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',
         # Audio Files
         'source/dtmf_queue.cc',
         'source/dtmf_queue.h',
         'source/rtp_receiver_audio.cc',
         'source/rtp_receiver_audio.h',
         'source/rtp_sender_audio.cc',
         'source/rtp_sender_audio.h',
         # Video Files
@@ skipping 33 matching lines @@
         'source/vp8_partition_aggregator.h',
         # Mocks
         'mocks/mock_rtp_rtcp.h',
         'source/mock/mock_rtp_payload_strategy.h',
       ], # source
       # TODO(jschuh): Bug 1348: fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],
     },
   ],
 }