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

webrtc/test/vcm_capturer.cc

Index: webrtc/test/vcm_capturer.cc
diff --git a/webrtc/test/vcm_capturer.cc b/webrtc/test/vcm_capturer.cc
index 49d97101e9459318bf284c5f667ad23adec91a2f..c79f99ffbae86b916c793ac48f7a6e0db8fac413 100644
--- a/webrtc/test/vcm_capturer.cc
+++ b/webrtc/test/vcm_capturer.cc
@@ -78,7 +78,7 @@ void VcmCapturer::Stop() {
 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;
 }