Add information about microphone gain changes to AEC3

Changes in the microphone gain are effecting the AEC in the sense
that each change in the microphone gain is a change in the echo
path seen by the AEC. This CL utilizes the ability of AEC3 to
leverage information about known changes in the analog microphone
gain.

BUG=webrtc:6018

Review-Url: https://codereview.webrtc.org/2808073002
Cr-Commit-Position: refs/heads/master@{#17625}
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index 4e25f15..37b15c2 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -366,6 +366,8 @@
     // tracked by the capture_audio_.
     StreamConfig capture_processing_format;
     int split_rate;
+    int previous_agc_level;
+    bool echo_path_gain_change;
   } capture_ GUARDED_BY(crit_capture_);
 
   struct ApmCaptureNonLockedState {