APM reports ERL and ERLE metrics for AEC3.

The audio processing module reports the metrics 'echo return loss'
and 'echo return loss enhancement' for AEC3.

Bug: webrtc:8533
Change-Id: I166c504adf013d6cb5d6d3c9717d0622c3454bb7
Reviewed-on: https://webrtc-review.googlesource.com/24880
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20835}

modules/audio_processing/include/audio_processing.h

@@ -940,6 +940,14 @@ class EchoControl {
   // Processes the capture signal in order to remove the echo.
   virtual void ProcessCapture(AudioBuffer* capture, bool echo_path_change) = 0;
 
+  struct Metrics {
+    double echo_return_loss;
+    double echo_return_loss_enhancement;
+  };
+
+  // Collect current metrics from the echo controller.
+  virtual Metrics GetMetrics() const = 0;
+
   virtual ~EchoControl() {}
 };