Move smoothing filter to common audio.

webrtc/common_audio/BUILD.gn

Index: webrtc/common_audio/BUILD.gn
diff --git a/webrtc/common_audio/BUILD.gn b/webrtc/common_audio/BUILD.gn
index bfed73eeebb93d110c74cfb3248349c665313dd5..bd85e2e5b9ab814e98677a7edbf352dca643bdce 100644
--- a/webrtc/common_audio/BUILD.gn
+++ b/webrtc/common_audio/BUILD.gn
@@ -86,6 +86,8 @@ rtc_static_library("common_audio") {
     "signal_processing/splitting_filter.c",
     "signal_processing/sqrt_of_one_minus_x_squared.c",
     "signal_processing/vector_scaling_operations.c",
+    "smoothing_filter.cc",
+    "smoothing_filter.h",
     "sparse_fir_filter.cc",
     "sparse_fir_filter.h",
     "vad/include/vad.h",
@@ -109,6 +111,7 @@ rtc_static_library("common_audio") {
   ]
 
   deps = [
+    "../base:rtc_exp_filter",
     "../system_wrappers",
   ]
 
@@ -257,6 +260,7 @@ if (rtc_include_tests) {
       "ring_buffer_unittest.cc",
       "signal_processing/real_fft_unittest.cc",
       "signal_processing/signal_processing_unittest.cc",
+      "smoothing_filter_unittest.cc",
       "sparse_fir_filter_unittest.cc",
       "vad/vad_core_unittest.cc",
       "vad/vad_filterbank_unittest.cc",