Fix clang style warnings in webrtc/modules/audio_coding/neteq

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

Review URL: https://webrtc-codereview.appspot.com/44109004

Cr-Commit-Position: refs/heads/master@{#8960}
diff --git a/webrtc/modules/audio_coding/neteq/packet_buffer.h b/webrtc/modules/audio_coding/neteq/packet_buffer.h
index b9a1618..d2d429b 100644
--- a/webrtc/modules/audio_coding/neteq/packet_buffer.h
+++ b/webrtc/modules/audio_coding/neteq/packet_buffer.h
@@ -43,7 +43,7 @@
   virtual void Flush();
 
   // Returns true for an empty buffer.
-  virtual bool Empty() const { return buffer_.empty(); }
+  virtual bool Empty() const;
 
   // Inserts |packet| into the buffer. The buffer will take over ownership of
   // the packet object.
@@ -105,15 +105,11 @@
                                 uint32_t horizon_samples);
 
   // Discards all packets that are (strictly) older than timestamp_limit.
-  virtual int DiscardAllOldPackets(uint32_t timestamp_limit) {
-    return DiscardOldPackets(timestamp_limit, 0);
-  }
+  virtual int DiscardAllOldPackets(uint32_t timestamp_limit);
 
   // Returns the number of packets in the buffer, including duplicates and
   // redundant packets.
-  virtual int NumPacketsInBuffer() const {
-    return static_cast<int>(buffer_.size());
-  }
+  virtual int NumPacketsInBuffer() const;
 
   // Returns the number of samples in the buffer, including samples carried in
   // duplicate and redundant packets.