Start NACKing as soon as we have the first packet of a key frame.

BUG=1605

Review URL: https://webrtc-codereview.appspot.com/1307007

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3855 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_coding/main/source/frame_buffer.h

@@ -44,6 +44,7 @@ public:
 
     bool IsRetransmitted() const;
     bool IsSessionComplete() const;
+    bool HaveFirstPacket() const;
     bool HaveLastPacket() const;
     // Makes sure the session contain a decodable stream.
     void MakeSessionDecodable();