Refactor VideoDenoiser to work with I420Buffer, not VideoFrame.

webrtc/common_video/include/video_frame_buffer.h

Index: webrtc/common_video/include/video_frame_buffer.h
diff --git a/webrtc/common_video/include/video_frame_buffer.h b/webrtc/common_video/include/video_frame_buffer.h
index 6f082dee9bae928fbb005027b7262f03bdde3053..e99642e562bd1a3a1a19237c78a87eb05b272004 100644
--- a/webrtc/common_video/include/video_frame_buffer.h
+++ b/webrtc/common_video/include/video_frame_buffer.h
@@ -122,6 +122,10 @@ class I420Buffer : public VideoFrameBuffer {
   static rtc::scoped_refptr<I420Buffer> Copy(
       const rtc::scoped_refptr<VideoFrameBuffer>& buffer);
 
+  // Create a new buffer with identical strides, and copy the pixel data.
+  static rtc::scoped_refptr<I420Buffer> CopyKeepStride(
+      const rtc::scoped_refptr<VideoFrameBuffer>& buffer);
+
  protected:
   ~I420Buffer() override;