Removed the dependency in GainControlImpl on the ProcessingComponent class

BUG=webrtc:5353

Review URL: https://codereview.webrtc.org/1768943002

Cr-Commit-Position: refs/heads/master@{#11949}
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index d79d34c..20ca3a4 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -198,6 +198,8 @@
       EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializeEchoCanceller()
       EXCLUSIVE_LOCKS_REQUIRED(crit_render_, crit_capture_);
+  void InitializeGainController()
+      EXCLUSIVE_LOCKS_REQUIRED(crit_render_, crit_capture_);
   void InitializeEchoControlMobile()
       EXCLUSIVE_LOCKS_REQUIRED(crit_render_, crit_capture_);
   int InitializeLocked(const ProcessingConfig& config)