PacketBuffer is now ref counted.

Since all FrameObjects have a reference to its PacketBuffer and since
the PacketBuffer can be thrown away at any moment the PacketBuffer
has to be ref counted in order to avoid FrameObjects dereferencing a potentially
destroyed object.

BUG=webrtc:5514
R=danilchap@webrtc.org, mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2199133004 .

Cr-Commit-Position: refs/heads/master@{#13725}

webrtc/modules/video_coding/frame_object.h

@@ -75,7 +75,7 @@ class RtpFrameObject : public FrameObject {
   RTPVideoTypeHeader* GetCodecHeader() const;
 
  private:
-  PacketBuffer* packet_buffer_;
+  rtc::scoped_refptr<PacketBuffer> packet_buffer_;
   enum FrameType frame_type_;
   VideoCodecType codec_type_;
   uint16_t first_seq_num_;