Make the nonlinear beamformer steerable

Depends on this CL: https://codereview.webrtc.org/1395453004/

R=andrew@webrtc.org

Review URL: https://codereview.webrtc.org/1394103003 .

Cr-Commit-Position: refs/heads/master@{#10458}

webrtc/modules/audio_processing/audio_processing_impl.h

@@ -208,6 +208,7 @@ class AudioProcessingImpl : public AudioProcessing {
   const bool beamformer_enabled_;
   rtc::scoped_ptr<Beamformer<float>> beamformer_;
   const std::vector<Point> array_geometry_;
+  const SphericalPointf target_direction_;
 
   bool intelligibility_enabled_;
   rtc::scoped_ptr<IntelligibilityEnhancer> intelligibility_enhancer_;