Adding SmoothingFilter to audio network adaptor.

BUG=webrtc:6303

Review-Url: https://codereview.webrtc.org/2339523002
Cr-Commit-Position: refs/heads/master@{#14313}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index 4fe49e5..2c90c7a 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -710,6 +710,8 @@
     "audio_network_adaptor/dtx_controller.cc",
     "audio_network_adaptor/dtx_controller.h",
     "audio_network_adaptor/include/audio_network_adaptor.h",
+    "audio_network_adaptor/smoothing_filter.cc",
+    "audio_network_adaptor/smoothing_filter.h",
   ]
   configs += [ "../..:common_config" ]
   public_configs = [ "../..:common_inherited_config" ]