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/session_info.h

@@ -59,6 +59,7 @@ class VCMSessionInfo {
   // Returns the number of bytes deleted from the session.
   int MakeDecodable();
   int SessionLength() const;
+  bool HaveFirstPacket() const;
   bool HaveLastPacket() const;
   bool session_nack() const;
   webrtc::FrameType FrameType() const { return frame_type_; }