Explicitly set the observer to null in the dtor of MediaStreamRemoteVideoSource.

content/renderer/media/webrtc/media_stream_remote_video_source.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/webrtc/media_stream_remote_video_source.h"
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/debug/trace_event.h"
 #include "base/location.h"
@@ skipping 179 matching lines @@
 }
 
 MediaStreamRemoteVideoSource::MediaStreamRemoteVideoSource(
     const scoped_refptr<MediaStreamRemoteVideoSource::Observer>& observer)
     : observer_(observer), weak_factory_(this) {
   observer->SetSource(weak_factory_.GetWeakPtr());
 }
 
 MediaStreamRemoteVideoSource::~MediaStreamRemoteVideoSource() {
   DCHECK(CalledOnValidThread());
+  observer_ = nullptr;
 }
 
 void MediaStreamRemoteVideoSource::GetCurrentSupportedFormats(
     int max_requested_width,
     int max_requested_height,
     double max_requested_frame_rate,
     const VideoCaptureDeviceFormatsCB& callback) {
   DCHECK(CalledOnValidThread());
   media::VideoCaptureFormats formats;
   // Since the remote end is free to change the resolution at any point in time
@@ skipping 36 matching lines @@
     case webrtc::MediaStreamTrackInterface::kEnded:
       SetReadyState(blink::WebMediaStreamSource::ReadyStateEnded);
       break;
     default:
       NOTREACHED();
       break;
   }
 }
 
 }  // namespace content