[Mojo Video Capture] Stop service when last client disconnects.

media/capture/video/video_capture_device_client.cc

Index: media/capture/video/video_capture_device_client.cc
diff --git a/media/capture/video/video_capture_device_client.cc b/media/capture/video/video_capture_device_client.cc
index 8d95f3da6f05cfdd3992be1dbeb2da8bcb5faba1..89c3a8d6d807271cfa0736c69a4f943231bef62e 100644
--- a/media/capture/video/video_capture_device_client.cc
+++ b/media/capture/video/video_capture_device_client.cc
@@ -151,7 +151,8 @@ void VideoCaptureDeviceClient::OnIncomingCapturedData(
         !external_jpeg_decoder_initialized_) {
       external_jpeg_decoder_initialized_ = true;
       external_jpeg_decoder_ = jpeg_decoder_factory_callback_.Run();
-      external_jpeg_decoder_->Initialize();
+      if (external_jpeg_decoder_)
+        external_jpeg_decoder_->Initialize();
     }
   }