Allow webrtc::test::VcmCapturer to be updated with its current sink.

webrtc/test/vcm_capturer.cc

 /*
  *  Copyright (c) 2013 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.
  */
 
@@ skipping 60 matching lines @@
 }
 
 void VcmCapturer::Stop() {
   rtc::CritScope lock(&crit_);
   started_ = false;
 }
 
 void VcmCapturer::AddOrUpdateSink(rtc::VideoSinkInterface<VideoFrame>* sink,
                                   const rtc::VideoSinkWants& wants) {
   rtc::CritScope lock(&crit_);
-  RTC_CHECK(!sink_);
+  RTC_CHECK(!sink_ || sink_ == sink);
   sink_ = sink;
 }
 
 void VcmCapturer::RemoveSink(rtc::VideoSinkInterface<VideoFrame>* sink) {
   rtc::CritScope lock(&crit_);
   RTC_CHECK(sink_ == sink);
   sink_ = nullptr;
 }
 
 void VcmCapturer::Destroy() {
@@ skipping 12 matching lines @@
                                           const VideoFrame& frame) {
   rtc::CritScope lock(&crit_);
   if (started_ && sink_)
     sink_->OnFrame(frame);
 }
 
 void VcmCapturer::OnCaptureDelayChanged(const int32_t id, const int32_t delay) {
 }
 }  // test
 }  // webrtc