Refactor VideoDenoiser to work with I420Buffer, not VideoFrame.

BUG=webrtc:5921
R=jackychen@webrtc.org, marpan@webrtc.org

Review URL: https://codereview.webrtc.org/2005733003 .

Cr-Commit-Position: refs/heads/master@{#13179}

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_;