Fix clang style warnings in webrtc/modules/audio_coding

Mostly this consists of marking functions with override when
applicable, and moving function bodies from .h to .cc files.

BUG=163
R=henrik.lundin@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44979004

Cr-Commit-Position: refs/heads/master@{#8938}
diff --git a/webrtc/modules/audio_coding/main/acm2/nack.h b/webrtc/modules/audio_coding/main/acm2/nack.h
index 4224c99..1fb3e9b 100644
--- a/webrtc/modules/audio_coding/main/acm2/nack.h
+++ b/webrtc/modules/audio_coding/main/acm2/nack.h
@@ -59,7 +59,7 @@
   // Factory method.
   static Nack* Create(int nack_threshold_packets);
 
-  ~Nack() {}
+  ~Nack();
 
   // Set a maximum for the size of the NACK list. If the last received packet
   // has sequence number of N, then NACK list will not contain any element