Add _decoder CHECK to VCMGenericDecoder constructor.

webrtc/modules/video_coding/generic_decoder.h

Index: webrtc/modules/video_coding/generic_decoder.h
diff --git a/webrtc/modules/video_coding/generic_decoder.h b/webrtc/modules/video_coding/generic_decoder.h
index d79b5e59f22213995942d0cdbc86cc6abf95fb30..13759964ab16aafd0dc8b527ac018650ad76b855 100644
--- a/webrtc/modules/video_coding/generic_decoder.h
+++ b/webrtc/modules/video_coding/generic_decoder.h
@@ -65,7 +65,7 @@ class VCMGenericDecoder
 {
     friend class VCMCodecDataBase;
 public:
-    VCMGenericDecoder(VideoDecoder& decoder, bool isExternal = false);
+    VCMGenericDecoder(VideoDecoder* decoder, bool external);
     ~VCMGenericDecoder();
 
     /**
@@ -102,7 +102,7 @@ private:
     VCMDecodedFrameCallback*    _callback;
     VCMFrameInformation         _frameInfos[kDecoderFrameMemoryLength];
     uint32_t                    _nextFrameInfoIdx;
-    VideoDecoder&               _decoder;
+    VideoDecoder* const         _decoder;
     VideoCodecType              _codecType;
     bool                        _isExternal;
     bool                        _keyFrameDecoded;