Reduce digital adaptive AGC2 gain in some situations.

Hypothetical scenario: short weak speech at start of call, then high
noise. The digital adaptive AGC2 would pick a high gain, and then
continue to apply it on the noise. Unless the noise is detected by the
noise estimator, the gain would never be reduced.

This CL addresses the issue by sending limiter gain info to the
adaptive digital AGC2.

Bug: webrtc:7494
Change-Id: Idf5c2686af0f5e5bad981d39a95b8efc9ffb9d64
Reviewed-on: https://webrtc-review.googlesource.com/102641
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24922}

modules/audio_processing/gain_controller2.cc

@@ -43,7 +43,7 @@ void GainController2::Process(AudioBuffer* audio) {
   AudioFrameView<float> float_frame(audio->channels_f(), audio->num_channels(),
                                     audio->num_frames());
   if (adaptive_digital_mode_) {
-    adaptive_agc_.Process(float_frame);
+    adaptive_agc_.Process(float_frame, fixed_gain_controller_.LastAudioLevel());
   }
   fixed_gain_controller_.Process(float_frame);
 }