External VNR speed improvement.

Improved visual quality with 3x times speed-up.
Change list:
 1. Remove second chance filter in temporal denoising filter to mitigate trailing artifact.
 2. Add swap buffer to save one whole-frame memcpy.
 3. Do noise estimation on every N blocks.
 4. Adopt a faster moving object detection algorithm (change the structure).
 5. Refactor the for loops and PositionCheck().
 6. Refactor the function ReduceFalseDetection (RFD).
 7. Fix a bug in TrailingBlock() which causes a mismatch.
 8. Change unit test to support swap buffer test.
 9. Remove CopyMem8x8, use memcpy to copy U/V plane which can be optimized future.
 10. Remove DenoiseMetrics.

Review URL: https://codereview.webrtc.org/1871853003

Cr-Commit-Position: refs/heads/master@{#12340}
diff --git a/webrtc/modules/video_processing/frame_preprocessor.cc b/webrtc/modules/video_processing/frame_preprocessor.cc
index fd0d0ef..c204214 100644
--- a/webrtc/modules/video_processing/frame_preprocessor.cc
+++ b/webrtc/modules/video_processing/frame_preprocessor.cc
@@ -23,6 +23,7 @@
   ca_ = new VPMContentAnalysis(true);
   vd_ = new VPMVideoDecimator();
   EnableDenosing(false);
+  denoised_frame_toggle_ = 0;
 }
 
 VPMFramePreprocessor::~VPMFramePreprocessor() {
@@ -116,9 +117,18 @@
 
   const VideoFrame* current_frame = &frame;
   if (denoiser_) {
-    denoiser_->DenoiseFrame(*current_frame, &denoised_frame_,
-                            &denoised_frame_prev_, 0);
-    current_frame = &denoised_frame_;
+    VideoFrame* denoised_frame = &denoised_frame_[0];
+    VideoFrame* denoised_frame_prev = &denoised_frame_[1];
+    // Swap the buffer to save one memcpy in DenoiseFrame.
+    if (denoised_frame_toggle_) {
+      denoised_frame = &denoised_frame_[1];
+      denoised_frame_prev = &denoised_frame_[0];
+    }
+    // Invert the flag.
+    denoised_frame_toggle_ ^= 1;
+    denoiser_->DenoiseFrame(*current_frame, denoised_frame, denoised_frame_prev,
+                            true);
+    current_frame = denoised_frame;
   }
 
   if (spatial_resampler_->ApplyResample(current_frame->width(),