Fixes two bugs related to padding in the jitter buffer.

- Pad packets (empty) were often NACKed even though they were received.
- Padding only frames (empty) didn't properly update the decoding state,
  and would therefore be NACKed even though they were received.

TEST=trybots

BUG=1150

Review URL: https://webrtc-codereview.appspot.com/966026

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3181 4adac7df-926f-26a2-2b94-8c16560cd09d

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

@@ -33,6 +33,7 @@ class VCMDecodingState {
   void SetState(const VCMFrameBuffer* frame);
   // Set the decoding state one frame back.
   void SetStateOneBack(const VCMFrameBuffer* frame);
+  void UpdateEmptyFrame(const VCMFrameBuffer* frame);
   // Update the sequence number if the timestamp matches current state and the
   // sequence number is higher than the current one. This accounts for packets
   // arriving late.