Implement NullVideoDecoder to avoid crash on unsupported decoders.

webrtc/video_decoder.h

Index: webrtc/video_decoder.h
diff --git a/webrtc/video_decoder.h b/webrtc/video_decoder.h
index 3cd94e8270dfb906870e8428455a02bff9fba979..7d3f728ada9adff519757f831c16121928f94a4f 100644
--- a/webrtc/video_decoder.h
+++ b/webrtc/video_decoder.h
@@ -122,6 +122,30 @@ class VideoDecoderSoftwareFallbackWrapper : public webrtc::VideoDecoder {
   DecodedImageCallback* callback_;
 };
 
+// Video decoder class to be used for unknown codecs. Doesn't support decoding
+// but logs messages to LS_ERROR.
+class NullVideoDecoder : public VideoDecoder {
+ public:
+  NullVideoDecoder();
+
+  int32_t InitDecode(const VideoCodec* codec_settings,
+                     int32_t number_of_cores) override;
+
+  int32_t Decode(const EncodedImage& input_image,
+                 bool missing_frames,
+                 const RTPFragmentationHeader* fragmentation,
+                 const CodecSpecificInfo* codec_specific_info,
+                 int64_t render_time_ms) override;
+
+  int32_t RegisterDecodeCompleteCallback(
+      DecodedImageCallback* callback) override;
+
+  int32_t Release() override;
+  int32_t Reset() override;
+
+  const char* ImplementationName() const override;
+};
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_VIDEO_DECODER_H_