Adds new AEC API to audio_processing.

One unit test added.
Tested with audioproc_unittest and trybots

TEST=none
BUG=none

Review URL: https://webrtc-codereview.appspot.com/1154004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3613 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_processing/include/audio_processing.h

@@ -16,6 +16,8 @@
 #include "webrtc/modules/interface/module.h"
 #include "webrtc/typedefs.h"
 
+struct AecCore;
+
 namespace webrtc {
 
 class AudioFrame;
@@ -342,6 +344,12 @@ class EchoCancellation {
   // last call to |GetDelayMetrics()|.
   virtual int GetDelayMetrics(int* median, int* std) = 0;
 
+  // Returns a pointer to the low level AEC component.  In case of multiple
+  // channels, the pointer to the first one is returned.  A NULL pointer is
+  // returned when the AEC component is disabled or has not been initialized
+  // successfully.
+  virtual struct AecCore* aec_core() const = 0;
+
  protected:
   virtual ~EchoCancellation() {}
 };