Add the internals of RtcEvent's subclasses

webrtc/logging/rtc_event_log/events/rtc_event_logging_started.cc

 /*
  *  Copyright (c) 2017 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.
  */
 
 #include "webrtc/logging/rtc_event_log/events/rtc_event_logging_started.h"
 
 namespace webrtc {
 
+RtcEventLoggingStarted::RtcEventLoggingStarted(int64_t timestamp_us)
+    : timestamp_us_(timestamp_us) {}
+
 RtcEvent::Type RtcEventLoggingStarted::GetType() const {
   return RtcEvent::Type::LoggingStarted;
 }
 
 }  // namespace webrtc