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.cc b/webrtc/modules/audio_coding/neteq/dtmf_buffer.cc
index b07d561..24aa9fe 100644
--- a/webrtc/modules/audio_coding/neteq/dtmf_buffer.cc
+++ b/webrtc/modules/audio_coding/neteq/dtmf_buffer.cc
@@ -20,6 +20,16 @@
 
 namespace webrtc {
 
+DtmfBuffer::DtmfBuffer(int fs_hz) {
+  SetSampleRate(fs_hz);
+}
+
+DtmfBuffer::~DtmfBuffer() = default;
+
+void DtmfBuffer::Flush() {
+  buffer_.clear();
+}
+
 // The ParseEvent method parses 4 bytes from |payload| according to this format
 // from RFC 4733:
 //
@@ -173,6 +183,14 @@
   return false;
 }
 
+size_t DtmfBuffer::Length() const {
+  return buffer_.size();
+}
+
+bool DtmfBuffer::Empty() const {
+  return buffer_.empty();
+}
+
 int DtmfBuffer::SetSampleRate(int fs_hz) {
   if (fs_hz != 8000 &&
       fs_hz != 16000 &&