Refactor VideoDenoiser to use a buffer pool, replacing explicit double buffering.

Also improve denoiser to not assume identical stride of all involved
frames, and delete the no longer needed function I420Buffer::CopyKeepStride.

BUG=None

Review-Url: https://codereview.webrtc.org/2469763002
Cr-Commit-Position: refs/heads/master@{#14940}

webrtc/modules/video_processing/frame_preprocessor.h

@@ -61,13 +61,11 @@ class VPMFramePreprocessor {
   // we can compute new content metrics every |kSkipFrameCA| frames.
   enum { kSkipFrameCA = 2 };
 
-  rtc::scoped_refptr<I420Buffer> denoised_buffer_[2];
   VideoFrame denoised_frame_;
   VideoFrame resampled_frame_;
   VPMSpatialResampler* spatial_resampler_;
   VPMVideoDecimator* vd_;
   std::unique_ptr<VideoDenoiser> denoiser_;
-  uint8_t denoised_frame_toggle_;
   uint32_t frame_cnt_;
 };