Add unit test for stand-alone denoiser and fixed some bugs.

webrtc/modules/video_processing/frame_preprocessor.cc

Index: webrtc/modules/video_processing/frame_preprocessor.cc
diff --git a/webrtc/modules/video_processing/frame_preprocessor.cc b/webrtc/modules/video_processing/frame_preprocessor.cc
index 36e1b9bbfd6d314e4c4f22d60e1ed9ffe0573040..eca14a70038e68cfc20363fd2f2a9270f629ba01 100644
--- a/webrtc/modules/video_processing/frame_preprocessor.cc
+++ b/webrtc/modules/video_processing/frame_preprocessor.cc
@@ -22,7 +22,7 @@ VPMFramePreprocessor::VPMFramePreprocessor()
   ca_ = new VPMContentAnalysis(true);
   vd_ = new VPMVideoDecimator();
   if (enable_denoising_) {
-    denoiser_ = new VideoDenoiser();
+    denoiser_ = new VideoDenoiser(true);
   } else {
     denoiser_ = nullptr;
   }