Refactor VideoDenoiser to work with I420Buffer, not VideoFrame.

webrtc/modules/video_processing/frame_preprocessor.h

Index: webrtc/modules/video_processing/frame_preprocessor.h
diff --git a/webrtc/modules/video_processing/frame_preprocessor.h b/webrtc/modules/video_processing/frame_preprocessor.h
index 4ac6b76e83074f9e24d7ffbe16237a461a2d6a9f..47c17ec2e7792cb90267e69b6ff3801915601974 100644
--- a/webrtc/modules/video_processing/frame_preprocessor.h
+++ b/webrtc/modules/video_processing/frame_preprocessor.h
@@ -61,7 +61,8 @@ class VPMFramePreprocessor {
   // we can compute new content metrics every |kSkipFrameCA| frames.
   enum { kSkipFrameCA = 2 };
 
-  VideoFrame denoised_frame_[2];
+  rtc::scoped_refptr<I420Buffer> denoised_buffer_[2];
+  VideoFrame denoised_frame_;
   VideoFrame resampled_frame_;
   VPMSpatialResampler* spatial_resampler_;
   VPMVideoDecimator* vd_;