Allow disabling denoiser when it is enabled.

BUG=webrtc:5255

Review URL: https://codereview.webrtc.org/1571423003

Cr-Commit-Position: refs/heads/master@{#11312}
diff --git a/webrtc/modules/video_processing/frame_preprocessor.cc b/webrtc/modules/video_processing/frame_preprocessor.cc
index 6778a59..d91cacf 100644
--- a/webrtc/modules/video_processing/frame_preprocessor.cc
+++ b/webrtc/modules/video_processing/frame_preprocessor.cc
@@ -95,7 +95,11 @@
 }
 
 void VPMFramePreprocessor::EnableDenosing(bool enable) {
-  denoiser_.reset(new VideoDenoiser(true));
+  if (enable) {
+    denoiser_.reset(new VideoDenoiser(true));
+  } else {
+    denoiser_.reset();
+  }
 }
 
 const VideoFrame* VPMFramePreprocessor::PreprocessFrame(