Add _decoder CHECK to VCMGenericDecoder constructor.

webrtc/modules/video_coding/generic_decoder.h

 /*
  *  Copyright (c) 2012 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 47 matching lines @@
     VCMTiming& _timing;
     VCMTimestampMap _timestampMap;  // Guarded by |_critSect|.
     uint64_t _lastReceivedPictureID;
 };
 
 
 class VCMGenericDecoder
 {
     friend class VCMCodecDataBase;
 public:
-    VCMGenericDecoder(VideoDecoder& decoder, bool isExternal = false);
+    VCMGenericDecoder(VideoDecoder* decoder, bool external);
     ~VCMGenericDecoder();
 
     /**
     * Initialize the decoder with the information from the VideoCodec
     */
     int32_t InitDecode(const VideoCodec* settings,
                              int32_t numberOfCores);
 
     /**
     * Decode to a raw I420 frame,
@@ skipping 16 matching lines @@
     * Set decode callback. Deregistering while decoding is illegal.
     */
     int32_t RegisterDecodeCompleteCallback(VCMDecodedFrameCallback* callback);
 
     bool External() const;
 
 private:
     VCMDecodedFrameCallback*    _callback;
     VCMFrameInformation         _frameInfos[kDecoderFrameMemoryLength];
     uint32_t                    _nextFrameInfoIdx;
-    VideoDecoder&               _decoder;
+    VideoDecoder* const         _decoder;
     VideoCodecType              _codecType;
     bool                        _isExternal;
     bool                        _keyFrameDecoded;
 };
 
 }  // namespace webrtc
 
 #endif // WEBRTC_MODULES_VIDEO_CODING_GENERIC_DECODER_H_