Make interface destructor virtual

In summary, do this:

-  ~FrameCountObserver() {}
+  virtual ~FrameCountObserver() {}

BUG=
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4099005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5148 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/common_types.h

@@ -301,7 +301,7 @@ class BitrateStatisticsObserver {
 // Callback, used to notify an observer whenever frame counts have been updated
 class FrameCountObserver {
  public:
-  ~FrameCountObserver() {}
+  virtual ~FrameCountObserver() {}
   virtual void Notify(const unsigned int key_frames,
                       const unsigned int delta_frames,
                       const unsigned int ssrc) = 0;