Adding decoded_fec_rate to NetEQ Network Statistics.

A statistic is introduced to reflect the actual benefits of Opus FEC. It shows what percentage of samples in the rendered audio come from FEC data.

BUG=3867
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34969004

Cr-Commit-Position: refs/heads/master@{#8384}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8384 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/neteq/mock/mock_external_decoder_pcm16b.h

@@ -80,8 +80,6 @@ class MockExternalPcm16B : public ExternalPcm16B {
           uint32_t arrival_timestamp));
   MOCK_METHOD0(ErrorCode,
       int());
-  MOCK_CONST_METHOD0(codec_type,
-      NetEqDecoder());
 
  private:
   ExternalPcm16B real_;