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

The unit test will run the pure C denoiser and SSE2/NEON denoiser (based
on the CPU detection) and compare the denoised frames to ensure the bit
exact.

TBR=tommi@webrtc.org

BUG=webrtc:5255

Review URL: https://codereview.webrtc.org/1492053003

Cr-Commit-Position: refs/heads/master@{#11216}
diff --git a/webrtc/modules/video_processing/frame_preprocessor.cc b/webrtc/modules/video_processing/frame_preprocessor.cc
index db2b84b..6778a59 100644
--- a/webrtc/modules/video_processing/frame_preprocessor.cc
+++ b/webrtc/modules/video_processing/frame_preprocessor.cc
@@ -95,7 +95,7 @@
 }
 
 void VPMFramePreprocessor::EnableDenosing(bool enable) {
-  denoiser_.reset(new VideoDenoiser());
+  denoiser_.reset(new VideoDenoiser(true));
 }
 
 const VideoFrame* VPMFramePreprocessor::PreprocessFrame(