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/dtmf_buffer.h b/webrtc/modules/audio_coding/neteq/dtmf_buffer.h
index 5da3a16..861a948 100644
--- a/webrtc/modules/audio_coding/neteq/dtmf_buffer.h
+++ b/webrtc/modules/audio_coding/neteq/dtmf_buffer.h
@@ -55,14 +55,12 @@
   };
 
   // Set up the buffer for use at sample rate |fs_hz|.
-  explicit DtmfBuffer(int fs_hz) {
-    SetSampleRate(fs_hz);
-  }
+  explicit DtmfBuffer(int fs_hz);
 
-  virtual ~DtmfBuffer() {}
+  virtual ~DtmfBuffer();
 
   // Flushes the buffer.
-  virtual void Flush() { buffer_.clear(); }
+  virtual void Flush();
 
   // Static method to parse 4 bytes from |payload| as a DTMF event (RFC 4733)
   // and write the parsed information into the struct |event|. Input variable
@@ -82,9 +80,9 @@
   virtual bool GetEvent(uint32_t current_timestamp, DtmfEvent* event);
 
   // Number of events in the buffer.
-  virtual size_t Length() const { return buffer_.size(); }
+  virtual size_t Length() const;
 
-  virtual bool Empty() const { return buffer_.empty(); }
+  virtual bool Empty() const;
 
   // Set a new sample rate.
   virtual int SetSampleRate(int fs_hz);