audio_processing/agc: Put entire method set_output_will_be_muted() under lock

Setting the member value output_will_be_muted_ in set_output_will_be_muted() was done before the lock.
This caused a data race.

BUG=4477
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44929004

Cr-Commit-Position: refs/heads/master@{#8877}
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index 765cde7..500f08e 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -210,7 +210,7 @@
   int delay_offset_ms_;
   bool was_stream_delay_set_;
 
-  bool output_will_be_muted_;
+  bool output_will_be_muted_ GUARDED_BY(crit_);
 
   bool key_pressed_;