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 dc51164450ce6bc35d1cbbf0fcf737276123e2e8..dfffb9a0d6cae74c0eb9a844d7789f0fbd96ffcc 100644
--- a/webrtc/common_video/include/video_frame_buffer.h
+++ b/webrtc/common_video/include/video_frame_buffer.h
@@ -134,6 +134,10 @@ class I420Buffer : public VideoFrameBuffer {
   // Scale all of |src| to the size of |this| buffer, with no cropping.
   void ScaleFrom(const rtc::scoped_refptr<VideoFrameBuffer>& src);
 
+  // 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;