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}

webrtc/modules/audio_coding/main/interface/audio_coding_module.h

@@ -997,15 +997,7 @@ class ReceiverInfo;
 class AudioCoding {
  public:
   struct Config {
-    Config()
-        : neteq_config(),
-          clock(Clock::GetRealTimeClock()),
-          transport(NULL),
-          vad_callback(NULL),
-          play_dtmf(true),
-          initial_playout_delay_ms(0),
-          playout_channels(1),
-          playout_frequency_hz(32000) {}
+    Config();
 
     AudioCodingModule::Config ToOldConfig() const {
       AudioCodingModule::Config old_config;